Equipment Technology forth class By louay A. Mahdi ...uotechnology.edu.iq/dep-MechanicsandEquipment/Lectures and Syllab… · outer cover that acts like fin and the inner plastic

Equipment Technology forth class By louay A. Mahdi University of Technology Mechanical Eng.Dp. refrigeration and Air conditioning 2008 – 2009 Condenser and Evaporator 8

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Air-cooled condensers: As the name implies, in air-cooled condensers air is the external fluid, i.e., the refrigerant rejects heat to air flowing over the condenser. Air-cooled condensers can be further classified into natural convection type or forced convection type. Natural convection type: In natural convection type, heat transfer from the condenser is by buoyancy induced natural convection and radiation. Since the flow rate of air is small and the radiation heat transfer is also not very high, the combined heat transfer coefficient in these condensers is small. As a result a relatively large condensing surface is required to reject a given amount of heat. Hence these condensers are used for small capacity refrigeration systems like household refrigerators and freezers. The natural convection type condensers are either plate surface type or finned tube type. In plate surface type condensers used in small refrigerators and freezers, the refrigerant carrying tubes are attached to the outer walls of the refrigerator. The whole body of the refrigerator acts like a fin. Insulation is provided between the outer cover that acts like fin and the inner plastic cover of the refrigerator. It is for this reason that outer body of the refrigerator is always warm. Since the surface is warm, the problem of moisture condensation on the walls of the refrigerator does not arise in these systems. These condensers are sometimes called as flat back condensers. The finned type condensers are mounted either below the refrigerator at an angle or on the backside of the refrigerator. In case, it is mounted below, then the warm air rises up and to assist it an air envelope is formed by providing a jacket on backside of the refrigerator. The fin spacing is kept large to minimize the effect of fouling by dust and to allow air to flow freely with little resistance. In the older designs, the condenser tube (in serpentine form) was attached to a plate and the plate was mounted on the backside of the refrigerator. The plate acted like a fin and warm air rose up along it. In another common design, thin wires are welded to the serpentine tube coil. The wires act like fins for increased heat transfer area. Figure1 shows the schematic of a wire-and-tube type condenser commonly used in domestic refrigerators. Regardless of the type, refrigerators employing natural convection condenser should be located in such a way that air can flow freely over the condenser surface.

Fig.1: Schematic of a wire-and-tube type condenser used in small refrigeration systems


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Forced convection type: In forced convection type condensers, the circulation of air over the condenser surface is maintained by using a fan or a blower. These condensers normally use fins on air-side for good heat transfer. The fins can be either plate type or annular type. Figure. 2 show the schematic of a plate-fin type condenser. Forced convection type condensers are commonly used in window air conditioners, water coolers and packaged air conditioning plants. These are either chassis mounted or remote mounted. In chassis mounted type, the compressor, induction motor, condenser with condenser fan, accumulator, HP/LP cut- out switch and pressure gauges are mounted on a single chassis. It is called condensing unit of rated capacity. The components are matched to condense the required mass flow rate of refrigerant to meet the rated cooling capacity. The remote mounted type, is either vertical or roof mounted horizontal type. Typically the air velocity varies between 2 m/s to 3.5 m/s for economic design with airflow rates of 12 to 20 cmm per ton of refrigeration (TR). The air specific heat is 1.005 kJ/kg-K and density is 1.2 kg/m3. Therefore for 1 TR the temperature rise Δt

a = 3.5167/(1.2x1.005 x 16/60) = 10.9oC for average air flow rate of 16 cmm.

ence, the air temperature rises by 10 to 15oC as compared to 3 to 6oC for water in water cooled condensers. H The area of the condenser seen from outside in the airflow direction is called face area. The velocity at the face is called face velocity. This is given by the volume flow rate divided by the face area. The face velocity is usually around 2m/s to 3.5 m/s to limit the pressure drop due to frictional resistance. The coils of the tube in the flow direction are called rows. A condenser may have two to eight rows of the tubes carrying the refrigerant. The moist air flows over the fins while the refrigerant flows inside the tubes. The fins are usually of aluminum and tubes are made of copper. Holes of diameter slightly less than the tube diameter are punched in the plates and plates are slid over the tube bank. Then the copper tubes are pressurized which expands the tubes and makes a good thermal contact between the tube and fins. This process is also known as bulleting. For ammonia condensers mild steel tubes with mild steel fins are used. In this case the fins are either welded or galvanizing is done to make a good thermal contact between fin and tube. In case of ammonia, annular crimpled spiral fins are also used over individual tubes instead of flat-plate fins. In finned tube heat exchangers the fin spacing may vary from 3 to 7 fins per cm. The secondary surface area is 10 to 30 times the bare pipe area hence; the finned coils are very compact and have smaller weight.

Fig.2: Schematic of forced type condenser


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Water Cooled Condensers: In water cooled condensers water is the external fluid. Depending upon the construction, water cooled condensers can be further classified into: 1. Double pipe or tube-in-tube type 2. Shell-and-coil type 3. Shell-and-tube type Double Pipe or tube-in-tube type: Double pipe condensers are normally used up to 10 TR capacities. Figure 3 shows the schematic of a double pipe type condenser. As shown in the figure, in these condensers the cold water flows through the inner tube, while the refrigerant flows through the annulus in counter flow. Headers are used at both the ends to make the length of the condenser small and reduce pressure drop. The refrigerant in the annulus rejects a part of its heat to the surroundings by free convection and radiation. The heat transfer coefficient is usually low because of poor liquid refrigerant drainage if the tubes are long. Fig.3: Double pipe (tube-in-tube) type condenser Shell-and-coil type:

These condensers are used in systems up to 50 TR capacities. The water flows through multiple coils, which may have fins to increase the heat transfer coefficient. The refrigerant flows through the shell. In smaller capacity condensers, refrigerant flows through coils while water flows through the shell. Figure 4 shows a shell-and-coil type condenser. When water flows through the coils, cleaning is done by circulating suitable chemicals through the coils.

Fig.4: shell and coil type condenser


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Shell-and-tube type: This is the most common type of condenser used in systems from 2 TR up to thousands of TR capacity. In these condensers the refrigerant flows through the shell while water flows through the tubes in single to four passes. The condensed refrigerant collects at the bottom of the shell. The coldest water contacts the liquid refrigerant so that some sub cooling can also be obtained. The liquid refrigerant is drained from the bottom to the receiver. There might be a vent connecting the receiver to the condenser for smooth drainage of liquid refrigerant. The shell also acts as a receiver. Further the refrigerant also rejects heat to the surroundings from the shell. The most common type is horizontal shell type. A schematic diagram of horizontal shell-and-tube type condenser is shown in Fig. 5.

Vertical shell-and-tube type condensers are usually used with ammonia in large capacity systems so that cleaning of the tubes is possible from top while the plant is running.

Fig.5: A two-pass, shell-and-tube type condenser

Equipment Technology forth class By louay A. Mahdi University of Technology Mechanical Eng.Dp. refrigeration and Air conditioning Condenser and Evaporator 8

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2008 – 2009 Evaporative condensers:

In evaporative condensers, both air and water are used to extract heat from the condensing refrigerant. Figure 6 shows the schematic of an evaporative condenser. Evaporative condensers combine the features of a cooling tower and water-cooled condenser in a single unit. In these condensers, the water is sprayed from top part on a bank of tubes carrying the refrigerant and air is induced upwards. There is a thin water film around the condenser tubes from which evaporative cooling takes place. The heat transfer coefficient for evaporative cooling is very large. Hence, the refrigeration system can be operated at low condensing temperatures (about 11 to 13 K above the wet bulb temperature of air). The water spray countercurrent to the airflow acts as cooling tower. The role of air is primarily to increase the rate of evaporation of water.

The required air flow rates are in the range of 350 to 500 m3/h per TR of refrigeration capacity.

Evaporative condensers are used in medium to large capacity systems. These are normally cheaper compared to water cooled condensers, which require a separate cooling tower. Evaporative condensers are used in places where water is scarce. Since water is used in a closed loop, only a small part of the water evaporates. Make-up water is supplied to take care of the evaporative loss. The water consumption is typically very low, about 5 percent of an equivalent water cooled condenser with a cooling tower. However, since condenser has to be kept outside, this type of condenser requires a longer length of refrigerant tubing, which calls for larger refrigerant inventory and higher pressure drops. Since the condenser is kept outside, to prevent the water from freezing, when outside temperatures are very low, a heater is placed in the water tank. When outside temperatures are very low it is possible to switch-off the water pump and run only the blowers, so that the condenser acts as an air cooled condenser.

Another simple form of condenser used normally in older type cold storages is called as atmospheric condenser. The principle of the atmospheric condenser is similar to evaporative condenser, with a difference that the air flow over the condenser takes place by natural means as no fans or blowers are used. A spray system sprays water over condenser tubes. Heat transfer outside the tubes takes by both sensible cooling and evaporation, as a result the external heat transfer coefficient is relatively large. The condenser pipes are normally large, and they can be either horizontal or vertical. Though these condensers are effective and economical they are being replaced with other types of condensers due to the problems such as algae formation on condenser tubes, uncertainty due to external air circulation etc.

Fig.6: Schematic of evaporator condenser


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Air cooled vs. water cooled condensers: The Salient features of air cooled and water cooled condensers are shown below in Table 1. The advantages and disadvantages of each type are discussed below.

Parameter Air cooled Water cooled Temperature difference, T

C – T

coolant

o C 6 to 22 6 to 12

Volume flow rate of coolant per TR m3/min 12 to 20 0.007 to 0.02

Heat transfer area per TR m2 10 to 15 0.5 to 1.0

Face Velocity m/s 2.5 to 6 2 to 3

Fan or pump power per TR W 75 to 100 negligible

Table.1: Comparison between air cooled and water cooled condensers Advantages and disadvantages: Air-cooled condensers are simple in construction since no pipes are required for air. Further, the disposal of warm air is not a problem and it is available in plenty. The fouling of condenser is small and maintenance cost is low. However, since the specific heat of air is one fourth of that of water and density is one thousandth of that of water, volume flow rates required are very large. The thermal conductivity is small; hence heat transfer coefficient is also very small. Also, air is available at dry-bulb temperature while water is available at a lower temperature, which is 2 to 3 oC above the wet-bulb temperature. The temperature rise of air is much larger than that of water, therefore the condenser temperature becomes large and COP reduces. Its use is normally restricted to 10 TR although blower power goes up beyond 5 TR. In systems up to 3 TR with open compressors it is mounted on the same chassis as the compressor and the compressor motor drives the condenser fan also. In middle-east countries where is shortage of fresh water these are used up to 100 TR or more. The air-cooled condensers cost two to three times more than water-cooled condensers. The water-cooled condenser requires cooling tower since water is scarce in municipality areas and has to be recycled. Water from lakes and rivers cannot be thrown back in warm state since it affects the marine life adversely. Increased first cost and maintenance cost of cooling tower offsets the cost advantage of water-cooled condenser. Fouling of heat exchange surface is a big problem in use of water.


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Optimum condenser pressure for lowest running cost : The total running cost of a refrigeration system is the sum of costs of compressor power and the cost of water. The cost of water can be the cost of municipal water or the cost of running a cooling tower. The compressor power increases as the condenser temperature or the pressure increases for fixed evaporator temperature. The water from a cooling tower is usually available at a fixed temperature equal to wet-bulb temperature of air plus the approach of the cooling tower. As the condenser temperature increases the overall log mean temperature difference increases, as a result lower mass flow rate of cooling water is required. This reduces the cost of water at higher condenser temperatures. Figure 7 shows the general trend of the total running cost of a refrigeration system. It is observed that there is a condenser pressure at which the running cost is minimum and it is recommended that the system should be run at this pressure. A complete analysis of the cost should actually be carried out which should include the first cost of the whole system, the interest on capital, the depreciation, the maintenance cost the operator cost etc. The final selection of the system and operating conditions should be such that the cost is the least over the running life of the system. Fig.7: Variation of total running cost of a refrigeration system with condensing pressure


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Equipment Technology forth class By louay A. Mahdi University of Technology Mechanical Eng.Dp. refrigeration and Air conditioning 2008 – 2009 Expansion devices 9

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Introduction An expansion device is another basic component of a refrigeration system. The basic functions of an expansion device used in refrigeration systems are to: 1. Reduce pressure from condenser pressure to evaporator pressure, and 2. Regulate the refrigerant flow from the high-pressure liquid line into the evaporator at a rate equal to the evaporation rate in the evaporator. Under ideal conditions, the mass flow rate of refrigerant in the system should be proportional to the cooling load. Sometimes, the product to be cooled is such that a constant evaporator temperature has to be maintained. In other cases, it is desirable that liquid refrigerant should not enter the compressor. In such a case, the mass flow rate has to be controlled in such a manner that only superheated Vapour leaves the evaporator. Again, an ideal refrigeration system should have the facility to control it in such a way that the energy requirement is minimum and the required criterion of temperature and cooling load are satisfied. Some additional controls to control the capacity of compressor and the space temperature may be required in addition, so as to minimize the energy consumption. The expansion devices used in refrigeration systems can be divided into fixed opening type or variable opening type. As the name implies, in fixed opening type the flow area remains fixed, while in variable opening type the flow area changes with changing mass flow rates. There are basically seven types of refrigerant expansion devices. These are:

1. Hand (manual) expansion valves 2. Capillary Tubes 3. Orifice 4. Constant pressure or Automatic Expansion Valve (AEV) 5. Thermostatic Expansion Valve (TEV) 6. Float type Expansion Valve: a) High Side Float Valve b) Low Side Float Valve 7. Electronic Expansion Valve

Of the above seven types, Capillary tube and orifice belong to the fixed opening type, while the rest belong to the variable opening type. Of the above seven types, the hand operated expansion valve is not used when an automatic control is required. The orifice type expansion is used only in some special applications (why?). Hence these two are not discussed here.


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Capillary Tube A capillary tube is a long, narrow tube of constant diameter. The word “capillary” is a misnomer since surface tension is not important in refrigeration application of capillary tubes. Typical tube diameters of refrigerant capillary tubes range from 0.5 mm to 3 mm and the length ranges from 1.0 m to 6 m. The pressure reduction in a capillary tube occurs due to the following two factors:

1. The refrigerant has to overcome the frictional resistance offered by tube walls. This leads to some pressure drop, and the liquid refrigerant flashes (evaporates) into mixture of liquid and Vapour as its pressure reduces. The density of Vapour is less than that of the liquid. Hence, the average density of refrigerant decreases as it flows in the tube. The mass flow rate and tube diameter (hence area) being constant, the velocity of refrigerant increases since m = ρVA. The increase in velocity or acceleration of the refrigerant also requires pressure drop.

2. Several combinations of length and bore are available for the same mass flow rate and pressure drop. However, once a capillary tube of some diameter and length has been installed in a refrigeration system, the mass flow rate through it will vary in such a manner that the total pressure drop through it matches with the pressure difference between condenser and the evaporator. Its mass flow rate is totally dependent upon the pressure difference across it; it cannot adjust itself to variation of load effectively.

Balance Point of Compressor and Capillary Tube The compressor and the capillary tube, under steady state must arrive at some suction and discharge pressures, which allows the same mass flow rate through the compressor and the capillary tube. This state is called the balance point. Condenser and evaporator pressures are saturation pressures at corresponding condenser and evaporator temperatures. Figure 9.1 shows the variation of mass flow rate with evaporator pressure through the compressor and the capillary tube for three values of condenser temperatures namely, 30, 40 and 50oC.

The mass flow rate through the compressor decreases if the pressure ratio increases since the volumetric efficiency of the compressor decreases with the increase of pressure ratio. The pressure ratio increases when either the evaporator pressure decreases or the condenser pressure increases. Hence, the mass flow rate through the compressor decreases with increase in condenser pressure and/or with decrease in evaporator pressure.

Fig.9.1 Variation of refrigerant mass flow rate through compressor and capillary tube with evaporator and condenser temperatures


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The pressure difference across the capillary tube is the driving force for the refrigerant to flow through it, hence mass flow rate through the capillary tube increases with increase in pressure difference across it. Thus the mass flow rate through the capillary tube increases as the condenser pressure increases and/or the evaporator pressure decreases. The variation of mass flow rate through capillary tube is shown for three condenser temperatures, namely, 30, 40 and 50oC in Figure 9.1

Effect Of load variation

The situation described above is in steady state. However, in practice the refrigeration load may vary due to several reasons, such as the variation of ambient temperatures etc. It is possible for the load to increase or decrease. This variation of load affects the operation of compressor and capillary tube and affects the balance point between them.

Selection of Capillary Tube

For any new system, the diameter and the length of capillary tube have to be selected by the designer such that the compressor and the capillary tube achieve the balanced point at the desired evaporator temperature.

There are analytical and graphical methods to select the capillary tube. The fine-tuning of the length is finally done by cut-and-try method. A tube longer than the design (calculated) value is installed with the expected result that evaporating temperature will be lower than expected. The tube is shortened until the desired balance point is achieved. This is done for mass production. If a single system is to be designed then tube of slightly shorter length than the design length is chosen. The tube will usually result in higher temperature than the design value. The tube is pinched at a few spots to obtain the required pressure and temperature.

Graphical Procedure A graphical procedure for capillary tube selection has been presented in ASHRAE Handbook. the mass flow rate of refrigerant through capillary tube at various inlet pressures, sub-cooling and dryness fraction through a capillary tube of 1.63 mm diameter and 2.03 m length.

These plots are for choked flow conditions. Corrections for non-choked flow conditions are given in ASHRAE Handbook.


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Advantages and disadvantages of capillary tubes Some of the advantages of a capillary tube are: 1. It is inexpensive. 2. It does not have any moving parts hence it does not require maintenance 3. Capillary tube provides an open connection between condenser and the evaporator hence during off-cycle, pressure equalization occurs between condenser and evaporator. This reduces the starting torque requirement of the motor since the motor starts with same pressure on the two sides of the compressor. Hence, a motor with low starting torque (squirrel cage Induction motor) can be used. 4. Ideal for hermetic compressor based systems, which are critically charged and factory assembled. Some of the disadvantages of the capillary tube are: 1. It cannot adjust itself to changing flow conditions in response to daily and seasonal variation in ambient temperature and load. Hence, COP is usually low under off design conditions. 2. It is susceptible to clogging because of narrow bore of the tube; hence, utmost care is required at the time of assembly. A filter-drier should be used ahead of the capillary to prevent entry of moisture or any solid particles

3. during off-cycle liquid refrigerant flows to evaporator because of pressure difference between condenser and evaporator. The evaporator may get flooded and the liquid refrigerant may flow to compressor and damage it when it starts. Therefore critical charge is used in capillary tube based systems. Further, it is used only with hermetically sealed compressors where refrigerant does not leak so that critical charge can be used. Normally an accumulator is provided after the evaporator to prevent slugging of compressor.


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Automatic Expansion Valve (AEV)

An Automatic Expansion Valve (AEV) also known as a constant pressure expansion valve acts in such a manner so as to maintain a constant pressure and thereby a constant temperature in the evaporator. The schematic diagram of the valve is shown in Fig.9.4 As shown in the figure; the valve consists of an adjustment spring that can be adjusted to maintain the required temperature in the evaporator.

When the refrigerant starts to enter the evaporator, the evaporator pressure does not decrease at the same fast rate as at starting time. Thus, the movement of the needle stand will slow down as the refrigerant starts entering. The constant is sum of force due to spring force and the atmospheric pressure; hence it depends upon position of adjustment spring. This will be the equilibrium position. Then onwards, the valve acts in such a manner that the evaporator pressure remains constant as long as the refrigeration load is constant. At this point, the mass flow rate through the valve is the same as that through the compressor. Effect of Load Variation The mass flow rate through the valve is directly proportional to the pressure drop through the orifice (P

c–P

e) and the

area of the orifice opening (needle position). At constant condenser pressure the mass flow rate will decrease if the evaporator pressure p

e increases or as the orifice opening becomes narrower.

Applications of automatic expansion valve The automatic expansion valves are used wherever constant temperature is required; for example, milk chilling units and water coolers where freezing is disastrous. In air-conditioning systems it is used when humidity control is by DX coil temperature. Automatic expansion valves are simple in design and are economical. These are also used in home freezers and small commercial refrigeration systems where hermetic compressors are used. Normally the usage is limited to systems of less than 10 TR capacities with critical charge. Critical charge has to be used since the system using AEV is prone to flooding. Hence, no receivers are used in these systems. In some valves a diaphragm is used in place of bellows.


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Thermostatic Expansion Valve (TEV) Thermostatic expansion valve is the most versatile expansion valve and is most commonly used in refrigeration systems. A thermostatic expansion valve maintains a constant degree of superheat at the exit of evaporator; hence it is most effective for dry evaporators in preventing the slugging of the compressors since it does not allow the liquid refrigerant to enter the compressor. The schematic diagram of the valve is given in Figure 9.5. This consists of a feeler bulb that is attached to the evaporator exit tube so that it senses the temperature at the exit of evaporator. The feeler bulb is connected to the top of the bellows by a capillary tube. The feeler bulb and the narrow tube contain some fluid that is called power fluid. The power fluid may be the same as the refrigerant in the refrigeration system, or it may be different. In case it is different from the refrigerant, then the TEV is called TEV with cross charge. The evaporator pressure is exerted below the bellows. In case the evaporator is large and has a significant pressure drop, the pressure from evaporator exit is fed directly to the bottom of the bellows by a narrow tube. This is called pressure-equalizing connection. Such a TEV is called TEV with external equalizer, otherwise it is known as TEV with internal equalizer. As the compressor is started, the evaporator pressure decreases at a very fast rate hence the force F

e decreases at a

very fast rate. This happens since TEV is closed and no refrigerant is fed to evaporator while compressors draw out refrigerant at a very fast rate and tries to evacuate the evaporator. The force F

p does not change during this period

since the evaporator temperature does not change. As the refrigerant enters the evaporator it arrests the fast rate of decrease of evaporator pressure. The movement of needle stand also slows down. The spring, however gets compressed as the needle stand moves downward to open the orifice It will be an ideal case if the degree of superheat is same at all evaporator temperatures for a given spring force. Effect of Load Variation The mass flow rate through the valve is directly proportional to the pressure drop through the orifice (P

c–P

e) and the

area of the orifice opening (needle position). At constant condenser pressure the mass flow rate will decrease if the evaporator pressure p

e increases or as the orifice opening becomes narrower.


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Applications of automatic expansion valve The automatic expansion valves are used wherever constant temperature is required; for example, milk chilling units and water coolers where freezing is disastrous. In air-conditioning systems it is used when humidity control is by DX coil temperature. Automatic expansion valves are simple in design and are economical. These are also used in home freezers and small commercial refrigeration systems where hermetic compressors are used. Normally the usage is limited to systems of less than 10 TR capacities with critical charge. Critical charge has to be used since the system using AEV is prone to flooding. Hence, no receivers are used in these systems. In some valves a diaphragm is used in place of bellows. TEV with External Pressure Equalizer

Hence a number of parallel paths or circuits are provided in the evaporator. The refrigerant is fed to these paths by a single TEV fitted with a distributor. In such a case, it is recommended that external pressure equalizer be used and care taken to ensure that all the paths are symmetric and have the same length.

EXTERNAL EQUALIZER The simple thermostatic expansion valve relies on the pressure under the diaphragm being approximately the same as that at the coil outlet, and small coil pressure drops can be accommodated by adjustments to the spring setting. Where an evaporator coil is divided into a number of parallel passes, a distribution device with a small pressure loss is used to ensure equal flow through each pass. Pressure drops of 1–2 bar are common. There will now be a much larger finite difference between the pressure under the diaphragm and that at the coil inlet. To correct for this, the body of the valve is modified to accommodate a middle chamber and an equalizing connection which is taken to the coil outlet, close to the phial position. Most thermostatic expansion valves have provision for an external equalizer connection The thermostatic expansion valve is substantially an un damped proportional control and hunts continuously, although the amplitude of this swing can be limited by correct selection and installation, and if the valve always works within its design range of mass flow. Difficulties arise when compressors are run at reduced load and the refrigerant mass flow falls below the valve design range. It is helpful to keep the condensing pressure steady, although it does not have to be constant and can usually be allowed to fall in colder weather to save compressor power. Valves on small systems may be seen to fully close and fully open at times. Excessive hunting of the thermostatic expansion valve means that the evaporator surface has an irregular refrigerant feed with a resulting slight loss of heat transfer effectiveness. If the hunting is caused by a time lag between the change of valve position and the effect at the evaporator outlet, a solution can be to increase the mass of the sensor phial which will increase damping. Over-sized valves and incorrect phial position can also give rise to hunting. The phial should always he located on the horizontal outlet, as close to the evaporator as possible and not on the underside of the pipe. Electronic expansion valves are now becoming increasingly popular for many applications.


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Advantages, disadvantages and applications of TEV The advantages of TEV compared to other types of expansion devices are: 1. It provides excellent control of refrigeration capacity as the supply of refrigerant to the evaporator matches the demand 2. It ensures that the evaporator operates efficiently by preventing starving under high load conditions 3. It protects the compressor from slugging by ensuring a minimum degree of superheat under all conditions of load, if properly selected. However, compared to capillary tubes and AEVs, a TEV is more expensive and proper precautions should be taken at the installation. For example, the feeler bulb must always be in good thermal contact with the refrigerant tube. The feeler bulb should preferably be insulated to reduce the influence of the ambient air. The bulb should be mounted such that the liquid is always in contact with the refrigerant tubing for proper control. The use of TEV depends upon degree of superheat. Hence, in applications where a close approach between the fluid to be cooled and evaporator temperature is desired, TEV cannot be used since very small extent of superheating is available for operation. A counter flow arrangement can be used to achieve the desired superheat in such a case. Alternately, a sub cooling HEX may be used and the feeler bulb mounted on the vapour exit line of the HEX. The valves with bellows have longer stroke of the needle, which gives extra sensitivity compared to diaphragm type of valve. But valves with bellows are more expensive. Thermostatic Expansion Valves are normally selected from manufacturers’ catalogs. The selection is based on the refrigeration capacity, type of the working fluid, operating temperature range etc. In practice, the design is different to suit different requirements such as single evaporators, multi-evaporators etc.


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Electronic Type Expansion Valve The schematic diagram of an electric expansion valve is shown in Fig.9.. As shown in the figure, an electronic expansion valve consists of an orifice and a needle in front it. The needle moves up and down in response to magnitude of current in the heating element. A small resistance allows more current to flow through the heater of the expansion valve, as a result the valve opens wider. A small negative coefficient thermistor is used if superheat control is desired. The thermistor is placed in series with the heater of the expansion valve. The heater current depends upon the thermistor resistance that depends upon the refrigerant condition. Exposure of thermistor to superheated Vapour permits thermistor to selfheat thereby lowering its resistance and increasing the heater current. This opens the valve wider and increases the mass flow rate of refrigerant. This process continues until the Vapour becomes saturated and some liquid refrigerant droplets appear. The liquid refrigerant will cool the thermistor and increase its resistance. Hence in presence of liquid droplets the thermistor offers a large resistance, which allows a small current to flow through the heater making the valve opening narrower. The control of this valve is independent of refrigerant and refrigerant pressure; hence it works in reverse flow direction also. It is convenient to use it in year-round-air-conditioning systems, which serve as heat pumps in winter with reverse flow. In another version of it the heater is replaced by stepper motor, which opens and closes the valve with a great precision giving a proportional control in response to temperature sensed by an element.

The electronic expansion valve offers a finer degree of control and system protection. The benefits can be summarized as: 1. Precise flow control over a wide range of capacities. 2. Rapid response to load changes. 3. Better control at low superheats so that less evaporator surface is required for superheat. More surface for evaporation results in higher evaporating temperature and better efficiency. 4. Electrical connection between components offers greater flexibility in system layout, which is important for compact systems. 5. The valve can close when the system shuts down, which eliminates the need for an additional shut off solenoid valve.


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Float type expansion valves: Float type expansion valves are normally used with flooded evaporators in large capacity refrigeration systems. A float type valve opens or closes depending upon the liquid level as sensed by a buoyant member, called as float. The float could take the form of a hollow metal or plastic ball, a hollow cylinder or a pan. Thus the float valve always maintains a constant liquid level in a chamber called as float chamber. Depending upon the location of the float chamber, a float type expansion valve can be either a low-side float valve or a high-side float valve. Low-side floats valves: A low-side float valve maintains a constant liquid level in a flooded evaporator or a float chamber attached to the evaporator. When the load on the system increases, more amount of refrigerant evaporates from the evaporator. As a result, the refrigerant liquid level in the evaporator or the low-side float chamber drops momentarily. The float then moves in such a way that the valve opening is increased and more amount of refrigerant flows into the evaporator to take care of the increased load and the liquid level is restored. The reverse process occurs when the load falls, i.e., the float reduces the opening of the valve and less amount of refrigerant flows into the evaporator to match the reduced load. As mentioned, these valves are normally used in large capacity systems and normally a by-pass line with a hand-operated expansion is installed to ensure system operation in the event of float failure. High-side float valves: Figure 24.18 shows the schematic of a high-side float valve. As shown in the figure, a high-side float valve maintains the liquid level constant in a float chamber that is connected to the condenser on the high pressure side. When the load increases, more amount of refrigerant evaporates and condenses. As a result, the liquid level in the float chamber rises momentarily. The float then opens the valve more to allow a higher amount of refrigerant flow to cater to the increased load, as a result the liquid level drops back to the original level. The reverse happens when the load drops. Since a high-side float valve allows only a fixed amount of refrigerant on the high pressure side, the bulk of the refrigerant is stored in the low-pressure side (evaporator). Hence there is a possibility of flooding of evaporator followed by compressor slugging. However, unlike low-side float valves, a high-side float valve can be used with both flooded as well as direct expansion type evaporators.

Figure. Low-pressure fl oat valves on flooded cooler


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Flow Rate through orifice In variable area type expansion devices, such as automatic and thermostatic expansion valves, the pressure reduction takes place as the fluid flows through an orifice of varying area. Let A

1and A

2 be the areas at the inlet and the outlet of

the orifice where, A1> A

2. Let V

1and V

2be the velocities, P

1 and

P2 are the pressures and ρ

1and ρ

2 be the densities at the inlet and outlet respectively of the orifice as shown in Figure

The area of the orifice opening is usually controlled to control the mass flow rate through the expansion valve. It is observed that the mass flow rate depends upon the difference between the condenser and evaporator pressures also. It is curious that single phase relations have been given above while it was shown that during expansion of high pressure liquid, the refrigerant flashes into a low pressure mixture of liquid and Vapour as it flows through the expansion valve. Actually, studies show that the refrigerant remains in a thermodynamic metastable liquid state as it flows through the orifice of the expansion valve. That is, it remains a liquid at a lower pressure and temperature during its passage through the orifice. It flashes into a mixture of liquid and Vapour as soon as it emerges out of the orifice of the valve. This kind of phenomenon has been observed in the initial sections of transparent capillary tubes also.

Equipment Technology forth class By louay A. Mahdi University of Technology Mechanical Eng.Dp. refrigeration and Air conditioning 2008 – 2009 Control devices 10

1

INTRODUCTION A refrigeration system can be built with only the four essential components: 1. Evaporator 2. Compressor 3. Condenser 4. Expansion valve Other system controls and components will, however, be necessary for safety, maintenance and control purposes. Figure 9.1 shows components associated with the compressor and Figure 9.2 shows some of the frequently used circuit components. Any pressure vessel in the circuit, depending on its size, may require a safety pressure relief device. Figure 10.1 Components associated with compressor Figure 10.2 Components for direct expansion circuits


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THERMOSTATS Since the purpose of refrigeration or air-conditioning system is to reduce or maintain temperature, a thermostat is usually fitted to stop the equipment or reduce its capacity when the required condition is reached. The following types are in use: 1. Movement of a bimetallic element 2. Expansion of a fluid 3. Vapour pressure of a volatile fluid The above produce a mechanical effect which can be used directly to operate an electric switch or modulate the pressure of an air jet (pneumatic system). 4. Electric resistance 5. Electronic – various types These last two produce an electric signal which must be measured and amplified to operate the controlled device. In order to prevent excessive cycling, a differential of say 2 K should be incorporated. HUMIDISTATS Where the equipment is required to maintain a predetermined level of humidity, a humidistat may be used instead of, or in addition to, a thermostat. The function will normally be to operate an electrical switch. Mechanical humidistats employ materials which change dimension with humidity, such as animal hair, plastics, celluloses, etc. These can work a switch directly. Electronic humidistats generally depend on the properties of a hygroscopic salt. The signal has to be measured and amplified.


3

PRESSURE SWITCHES The pressure in the condenser side of the system must always be limited to a maximum allowable value, and a pressure control is used to stop the compressor if necessary. High-pressure cut-outs, Figure 9.3, are fitted to all but the smallest of systems. Where a compressor is fitted with a shut-off valve, the pressure connection should be upstream of the valve. The compressor outlet pressure is brought to one side of a bellows or diaphragm, and balanced by an adjustable spring. A scale on the control indicates the pressure setting to commercial accuracy and is checked on commissioning the system. Figure10.3 Pressure switch (Danfoss) If the spring pressure is overcome, the switch will open and stop the compressor. The cut-out can also operate a warning. The cut-out point only needs to be some 2 bar higher than the expected summer operating pressure but there is a tendency to set such controls much higher – sometimes as much as 8 bar above summer pressures. At this setting, the user will not get a warning of abnormal running until the fault has reached serious proportions. Since excess pressure indicates malfunction of part of the system – usually a condenser fault or incorrect closure of a valve – the high-pressure switch should be reset manually, not automatically. Where the refrigeration system is providing an essential service which should not be interrupted, one high-pressure switch may be set at a warning level and operate an alarm, without stopping the compressor. A second switch, set somewhat higher, will stop the equipment if this warning is ignored and if excessive pressures are reached. All high-pressure cut-outs should be checked at least once a year, for correct setting and operation. Abnormally low suction pressures will lead to high discharge temperatures, owing to the high compression ratio, and possible malfunction of other components. Air cooling coils may frost excessively, or water chillers freeze. A low-pressure cut-out switch is usually fitted to stop the compressor under these circumstances. Settings may be 0.6–1.0 bar below the design evaporator pressures, but depend very much on the type of system. The cut-out setting should be above atmospheric pressure if possible to avoid the ingress of air through any leaks. Abnormally low pressure may not be an unsafe condition and the low pressure switch may be automatic reset, closing again at a pressure corresponding to a temperature just below that of the load. If a plant has been shut down long enough for all pressures to equalize and is then restarted, the suction pressure will pull down below normal until the liquid refrigerant has begun to circulate. Under such circumstances the low pressure switch may operate. This is a normal occurrence, but may require the addition of a delay timer to prevent frequent starting of the compressor motor. A low-pressure switch can also be used in conjunction with a thermostat and a solenoid valve in the pump-down circuit. In this method of control, the thermostat does not stop the compressor but de-energizes the liquid line solenoid valve to stop the supply of refrigerant to the evaporator. The compressor continues to run and pumps down the evaporator until stopped by the low pressure switch. When the thermostat again calls for cooling, it opens the solenoid valve, liquid enters the evaporator and the low-pressure switch will close again to restart the compressor. This method is used to ensure that the evaporator is kept clear of liquid when the plant is off. If there is any leak at the solenoid valve, it will cause the compressor to restart periodically to remove the surplus liquid from the coil (see Figure 10.4 ). Pressure switches are also made in miniature encapsulated versions, mainly pre-set for use in integrated control circuits.


4

OIL PRESSURE SWITCHES All compressors except the smallest have mechanical lubrication and will fail if the oil pressure falls because of a pump fault or oil shortage. Where an oil pump is used a cut-out, which will stop the compressor in the event of inadequate oil pressure, is provided. This takes the form of a differential pressure switch with a starting time delay. Since the oil pump inlet is at sump (suction) pressure, a pressure gauge on the pump discharge will indicate the total pressure at that point above atmospheric, i.e. suction (gauge) plus pump head. Any detection element for true oil pump pressure must sense both suction and pump outlet pressures and transducer the difference. Oil pressure cut-outs have connections to both sides of the oil pump and two internal bellows are opposed to measure the difference. Since there will be no oil pressure at the moment of starting, a time delay must be fitted to allow the oil pressure to build up. This timer may be thermal, mechanical or electrical. Operation of the oil cut-out indicates an unsafe condition and such controls are made with hand reset switches. Contacts on the switch can be used to operate an alarm to warn of the malfunction. Several compressor manufacturers offer electronic oil protection systems which provide more functionality, whilst retaining the option of hand re-set. Figure 10.4 The pump down circuit – the solenoid valve is thermostat controlled and compressor is low pressure controlled oil pressure, is provided. This takes the form of a differential pressure switch with a starting time delay. Since the oil pump inlet is at sump (suction) pressure, a pressure gauge on the pump discharge will indicate the total pressure at that point above atmospheric, i.e. suction (gauge) plus pump head. Any detection element for true oil pump pressure must sense both suction and pump outlet pressures and transducer the difference. Oil pressure cut-outs have connections to both sides of the oil pump and two internal bellows are opposed to measure the difference. Since there will be no oil pressure at the moment of starting, a time delay must be fitted to allow the oil pressure to build up. This timer may be thermal, mechanical or electrical. Operation of the oil cut-out indicates an unsafe condition and such controls are made with hand reset switches. Contacts on the switch can be used to operate an alarm to warn of the malfunction. Several compressor manufacturers offer electronic oil protection systems which provide more functionality, whilst retaining the option of hand re-set.


5

PRESSURE GAUGES Direct indication of the operating conditions of a compressor is by pressure gauges at suction, discharge and oil delivery. Such gauges are mounted on or near the compressor. Since the pressure losses along the discharge and suction lines are comparatively small on most systems, these pressures will also approximate to the conditions in the condenser and evaporator, and the equivalent saturation temperatures will be the condensing and evaporating temperatures. To indicate these temperatures for the common refrigerants, pressure gauges will have further calibrations showing these equivalent temperatures (see Figure 10.5 ). Figure 10.5 Refrigeration pressure gauges (Star Instruments) Gauge mechanisms are mostly of the bourdon tube type, having a flattened tube element, which distorts under pressure change. Gas pulsations from the compressor will be transmitted along the short connecting tubes and may lead to early failure of the needle mechanism. These can be damped by restricting the tube with a valve or orifice, or oil filling the gauge as shown in Figure 10.5, or both. Gauge needles should not be allowed to flicker noticeably from gas Pulsations. Pressure transducers are now used for integrated control circuits.


6

SOLENOID VALVES Electrically operated shut-off valves (Figure 10.6) are required for refrigerant and other circuits. These take the form of a plunger operated by a solenoid and working directly on the valve orifice or through a servo. The usual arrangement is to energize the solenoid to open the valve and de-energize to close. Sizes up to 50 mm bore tube connections are made. Beyond this, the solenoid acts as a pilot to a main servo (see Figure 10.7). Solenoid valves are used in refrigeration and air-conditioning systems for refrigerant lines, oil pressure pipes (to control oil return and capacity reducers), and water and compressed air lines. Figure 10.6 Solenoid valve (Danfoss) Figure 10.7 Evaporator pressure regulator valve (Danfoss)


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EVAPORATOR PRESSURE REGULATION VALVES Evaporator pressure regulation (EPR) valves (Figure 10.7) can be used in the suction line, and their function is to prevent the evaporator pressure falling below a predetermined or controlled value, although the compressor suction pressure may be lower. The application of a EPR valve is to: 1. Prevent damage to a liquid chilling evaporator which might result from freezing of the liquid. 2. Prevent frost forming on an air cooling evaporator, where this is close to freezing point, or where a temporary malfunction cannot be permitted to interrupt operation. 3. Permit two or more evaporators, working at different load temperatures, to work with the same compressor. 4. Modulate the evaporator pressure according to a varying load, controlled by the load temperature. 5. Act as a solenoid valve, controlled by a pilot solenoid valve. The simplest EPR valve is spring-loaded, balancing the thrust of the spring, plus atmospheric pressure, on one side of a diaphragm or piston, against the inlet or evaporator pressure. For working pressures below atmospheric, a helper spring is fitted below the diaphragm. Slight variations will result from changes in atmospheric pressure, but these are too small to materially affect a refrigeration control system. A service gauge is usually fitted adjacent to the valve or as part of the valve assembly, to facilitate setting or readjustment. Above about 40 mm pipe size, the basic EPR valve is used as a pilot to operate a main servo valve. Other pilot signals can be used on the same servo and many control functions are possible. Figure 10.7 shows a main servo controlled by an electric pilot which responds to a signal from a controller to maintain a constant load temperature to a high level of accuracy. The constant pressure pilot prevents the evaporator temperature becoming too low and the solenoid pilot is for on/off.


8

HOT GAS BY-PASS VALVES Where a compressor does not have any capacity reduction device and on–off switching will not give the degree of control required by the process, the cooling capacity can be regulated by injecting discharge gas back into the suction (see Figure 10.2 ). It has the effect of keeping the evaporator pressure constant, regardless of the load, and can have a wide range of capacity reduction, down to 10% of full load. It is a constant pressure valve, balancing the suction pressure against a pre-set spring. They may be either direct or pilot operated. They are often equipped with an external equalizer which works just like an externally equalized expansion valve. Injecting the hot gas upstream of the evaporator enables the expansion valve to open and admit more cool refrigerant which maintains gas velocity in the evaporator and aids oil return. Note the solenoid valve in Figure 10.2. If the system is set up with a pump down, then this solenoid must be wired in parallel with the pump down solenoid in order to achieve pump down. This control method is wasteful of energy. Hot gas bypass using type LCL liquid injection valve. Direct acting hot gas regulator admitting flow between TEV and venturi distributor.


9

SHUT-OFF VALVES Manual stop valves are required throughout a circuit to permit isolation during partial operation, service or maintenance (see Figures 10.8 and 10.9).

Figure10.8 Seal cap shut-off valve (Henry) Figure10.9 Ball valve (Henry) Small valves which are to be operated frequently have a pack less gland, either a diaphragm or bellows, and a hand wheel. Valves of all sizes which are only used occasionally will be sealed with ‘ O ’ rings. As a safeguard against leakage, they have no hand wheel fitted and the stem is provided with a covering cap which is only removed when the valve is to be operated. The stem will have flats for operation by a spanner. Most such valves can be back-seated to permit changing the ‘ O ’ rings. Valves should not be installed with the stem downwards, as any internal dirt will fall into the spindle thread. Under low-temperature conditions, ice will form on the spindle and will be forced into the gland if the valve is operated quickly. Under such circumstances, the spindle should be well greased, or the ice melted off first. Service stop valves on small compressors will usually carry a connection for a pressure cut-out or gauge, or for the temporary fitting of gauges or charging lines when servicing. The valve backseats to close off this port while gauges are being fitted. Valve seats are commonly of soft metal or of a resistant plastic such as PTFE. Ball valves, Figure 10.9 , are sometimes used for secure isolation of sections of circuits and they give less pressure drop when open.


10

FILTER-DRIERS With the halocarbons, it is essential to reduce the water content of the refrigerant circuit to a minimum by careful drying of components and the fitting of drying agents in the system. The common form of drier is a capsule charged with a solid desiccant such as activated alumina or zeolite (molecular sieve), and located in the liquid line ahead of the expansion valve. These capsules must have strainers to prevent loss of the drying agent into the circuit, and so form an effective filter-drier to also protect the valve orifice from damage by fine debris, Figure 10.10. Figure10.10 Filter drier (Henry) Large driers are made so they can be opened, and the spent drying agent removed and replaced with new. Small sizes are throwaway. Driers may also be used in the suction line. Suction line filter-driers are a temporary installation to clean up a system after repairs, and should be removed if the pressure drop becomes 1K greater than when new.


11

SIGHT GLASSES Sight glasses can be used to indicate whether gas is present in a pipe which should be carrying only liquid. The main application in refrigeration is in the liquid line from the receiver to the expansion valve. Close to the expansion valve is best because observation here can determine liquid presence at the valve, and being downstream of the filter drier, it can be used to indicate the need for filter change. If the equipment is running correctly, only liquid will be present and any gas bubbles seen will indicate a refrigerant shortage. Sight glasses for the halocarbons are commonly made of brass, and should have solder connections, Figure 10.11. For ammonia, they are made of steel or cast iron. Figure10.11 Sight glass/moisture indicator (Henry) Since the interior of the system can be seen at this point, advantage is taken in most types to insert a moisture-sensitive chemical which will indicate an excess of water by a change of colour. When such an indication is seen, the drier needs changing or recharging, and the colour should then revert to the ‘ dry ’ shade.


12

SUCTION ACCUMULATORS Suction line accumulators are sometimes inserted in halocarbon circuits, to serve the purpose of separating return liquid and prevent it passing over to the compressor. Since this liquid will be carrying oil, and this oil must be returned to the compressor, the outlet pipe within the separator dips to the bottom of this vessel and has a small bleed hole, to suck the oil out (see Figure 10.12 ). A version with a heating coil through which hot gas is passed can speed the evaporation of trapped liquid, Figure 10.12(b). Figure10.12 (a) suction line accumulator, (b) with heating coil (Henry)


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RELIEF VALVES Under several possible conditions of malfunction, high pressures can occur in parts of the system and mechanical relief devices are advised or mandatory. The requirements which must be observed are detailed in European Safety Standard EN378 (2008) and the Institute of Refrigeration Safety Codes. The standard form of relief valve is a spring-loaded plunger valve. Plunger-type relief valves, if located outdoors, should be protected from the ingress of rain, which may corrode the seat. Steel valves, when installed, should have a little oil poured in to cover the seat as rust protection. To prevent overpressure within a compressor, a relief valve or bursting disc is often fitted between the inlet and discharge connections. STRAINERS Piping circuits will usually contain a small quantity of dirt, scale and swarf, no matter what care is taken to keep these out. A strainer is fitted in the compressor suction to trap such particles before they can enter the machine. Such strainers are of metal mesh and will be located where they can be removed for cleaning. In some configurations two strainers may be fitted. As an extra safeguard, on new compressors a fabric liner may be fitted inside the mesh strainer to catch fine dirt which will be present. Such liners must be removed at the end of the running-in period, as they create a high resistance to gas flow. Oil strainers may be of metal mesh and within the sump, in which case the sump must be opened for cleaning. Self-cleaning disc strainers are also used, the dirt falling into a drain pot or into the sump itself. There is an increasing tendency to provide replaceable fabric oil filters external to the compressor body, following automobile practice. CHECK VALVES Non-return or check valves can be found in the following positions: 1. On reversible heat pump circuits, to prevent flow through expansion valves which are not in service on one cycle? 2. On hot gas circuits, to prevent the gas entering another evaporator 3. Where several compressors discharge into one condenser, to prevent liquid condensing back to an idle compressor 4. Where two or more evaporators work at different pressures, to prevent suction gas fl owing back to the colder ones

Equipment Technology forth class By louay A. Mahdi University of Technology Mechanical Eng.Dp. refrigeration and Air conditioning 2008 – 2009 Cooling towers 11

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2

MOST air-conditioning systems and industrial processes generate heat that must be removed and dissipated. Water is commonly used as a heat transfer medium to remove heat from refrigerant condensers or industrial process heat exchangers. In the past, this was accomplished by drawing a continuous stream of water from a utility water supply or a natural body of water, heating it as it passed through the process, and then discharging the water directly to a sewer or returning it to the body of water. Water purchased from utilities for this purpose has now become prohibitively expensive because of increased water supply and disposal costs. Similarly, cooling water drawn from natural sources is relatively unavailable because the ecological disturbance caused by the increased temperature of discharge water has become unacceptable. Air-cooled heat exchangers cool water by rejecting heat directly to the atmosphere, but the first cost and fan energy consumption of these devices are high and the plan area required is relatively large. They can economically cool water to within approximately 11 K of the ambient dry-bulb temperature—too high for the cooling water requirements of most refrigeration systems and many industrial processes. Cooling towers overcome most of these problems and therefore are commonly used to dissipate heat from water-cooled refrigeration, air-conditioning, and industrial process systems. The water consumption rate of a cooling tower system is only about 5% of that of a once-through system, making it the least expensive system to operate with purchased water supplies. Additionally, the amount of heated water discharged (blow down ) is very small, so the ecological effect is greatly reduced. Lastly, cooling towers can cool water to within 2 to 3 K of the ambient wet-bulb temperature, or about 20 K lower than can air-cool systems of reasonable size. This lower temperature improves the efficiency of the overall system, thereby reducing energy use significantly and increasing process output. PRINCIPLE OF OPERATION A cooling tower cools water by a combination of heat and mass transfer. Water to be cooled is distributed in the tower by spray nozzles, splash bars, or film-type fill, which exposes a very large water surface area to atmospheric air. Atmospheric air is circulated by (1) fans, (2) convective currents, (3) natural wind currents, (4) induction effect from sprays. A portion of the water absorbs heat to change from a liquid to a vapor at constant pressure. This heat of vaporization at atmospheric pressure is transferred from the water remaining in the liquid state into the air stream. Figure 1 shows the temperature relationship between water and air as they pass through a counter flow cooling tower. The curves indicate the drop in water temperature (A to B) and the rise in the air wet-bulb temperature (C to D) in their respective passages through the tower. The temperature difference between the water entering and leaving the cooling tower (A minus B) is the range. For a steady-state system, the range is the same as the water temperature rise through the load heat exchanger, provided the flow rate through the cooling tower and heat exchanger are the same. Accordingly, the range is determined by the heat load and water flow rate, not by the size or thermal capability of the cooling tower. The difference between the leaving water temperature and entering air wet-bulb temperature (B minus C) in Figure 1 is the approach to the wet bulb or simply the approach of the cooling tower. The approach is a function of cooling tower capability, and a larger cooling tower produces a closer approach (colder leaving water) for a given heat load, flow rate, and entering air condition. Thus, the amount of heat transferred to the atmosphere by the cooling tower is always equal to the heat load imposed on the tower, whereas the temperature level at which the heat is transferred is determined by the thermal capability of the cooling tower and the entering air wet-bulb temperature. Thermal performance of a cooling tower depends principally on the entering air wet-bulb temperature. The entering air dry-bulb temperature and relative humidity, taken independently, have an insignificant effect on thermal performance of mechanical-draft cooling towers, but do affect the rate of water evaporation in the cooling tower. A psychometric analysis of the air passing through a cooling tower illustrates this effect (Figure 2). Air enters at the ambient condition Point A, absorbs heat and mass (moisture) from the water, and exits at Point B in a saturated condition (at very light loads, the discharge air may not be fully saturated). The amount of heat transferred from the water to the air is proportional to the difference in enthalpy of the air between the entering and leaving conditions (hB − hA). Because lines of constant enthalpy correspond almost exactly to lines of constant wet-bulb temperature, the change in enthalpy of the air may be determined by the change in wet-bulb temperature of the air.


3

Air heating (Vector AB in Figure 2) may be separated into component AC, which represents the sensible portion of the heat absorbed by the air as the water is cooled, and component CB, which represents the latent portion. If the entering air condition is changed to Point D at the same wet-bulb temperature but at a higher dry-bulb temperature, the total heat transfer (Vector DB) remains the same, but the sensible and latent components change dramatically. DE represents sensible cooling of air, while EB represents latent heating as water gives up heat and mass to the air. Thus, for the same water-cooling load, the ratio of latent to sensible heat transfer can vary significantly. The ratio of latent to sensible heat is important in analyzing water usage of a cooling tower. Mass transfer (evaporation) occurs only in the latent portion of heat transfer and is proportional to the change in specific humidity. Because the entering air dry-bulb temperature or relative humidity affects the latent to sensible heat transfer ratio, it also affects the rate of evaporation. In Figure 2, the rate of evaporation in Case AB (WB − WA) is less than in Case DB (WB − WD) because the latent heat transfer (mass transfer) represents a smaller portion of the total. The evaporation rate at typical design conditions is approximately 1% of the water flow rate for each 7 K of water temperature range; however, the average evaporation rate over the operating season is less than the design rate because the sensible component of total heat transfer increases as entering air temperature decreases. In addition to water loss from evaporation, losses also occur because of liquid carryover into the discharge air stream and blow down to maintain acceptable water quality.


4

DESIGN CONDITIONS The thermal capability of any cooling tower may be defined by the following parameters: • Entering and leaving water temperatures • Entering air wet-bulb or entering air wet-bulb and dry-bulb temperatures • Water flow rate The entering air dry-bulb temperature affects the amount of water evaporated from any evaporative cooling tower. It also affects airflow through hyperbolic towers and directly establishes thermal capability in any indirect-contact cooling tower component operating in a dry mode. TYPES OF COOLING TOWERS Two basic types of evaporative cooling devices are used. The first of these, the direct-contact or open cooling tower (Figure 3), exposes water directly to the cooling atmosphere, thereby transferring the source heat load directly to the air. The second type, often called a closed-circuit cooling tower , involves indirect contact between heated fluid and atmosphere (Figure 4), essentially combining a heat exchanger and cooling tower into one relatively compact device. Of the direct-contact devices, the most rudimentary is a spray filled tower that exposes water to the air without any heat transfer medium or fill. In this device, the amount of water surface exposed to the air depends on the spray efficiency, and the time of contact depends on the elevation and pressure of the water distribution system.


5

Types of Direct-Contact Cooling Towers Non-Mechanical-Draft Towers. Aspirated by sprays or a differential in air density, these towers do not contain fill and do not use a mechanical air-moving device. The aspirating effect of the water spray, either vertical (Figure 7) or horizontal (Figure 8), induces airflow through the tower in a parallel flow pattern. Because air velocities for the vertical spray tower (both entering and leaving) are relatively low, such towers are susceptible to adverse wind effects and, therefore, are normally used to satisfy a low-cost requirement when operating temperatures are not critical to the system. Some horizontal spray towers (Figure 8) use high pressure sprays to induce large air quantities and improve air/water contact. Multi speed or staged pumping systems are normally recommended to reduce energy use in periods of reduced load and ambient conditions. Chimney (hyperbolic) towers have been used primarily for large power installations, but may be of generic interest (Figure 9). The heat transfer mode may be counter flow, cross-flow, or parallel flow. Air is induced through the tower by the air density differentials that exist between the lighter, heat-humidified chimney air and the outside atmosphere. Fill can be splash or film type. Primary justification of these high first-cost products comes through reduction in auxiliary power requirements (elimination of fan energy), reduced property area, and elimination of recirculation and/or vapor plume interference. Materials used in chimney construction have been primarily steel-reinforced concrete; early-day timber structures had size limitations. Mechanical-Draft Towers. Figure 10 shows five different designs for mechanical-draft (conventional) towers. Fans may be on the inlet air side (forced-draft) or the exit air side (induced-draft). The type of fan selected, either centrifugal or axial, depends on external pressure needs, permissible sound levels, and energy usage requirements. Water is down flow; the air may be up flow (counter flow heat transfer) or horizontal flow (cross-flow heat transfer). Air entry may be through one, two, three, or all four sides of the tower. All four combinations (i.e., forced-draft counter flow, induced-draft counter flow, forced-draft cross-flow, and induced-draft cross-flow) have been produced in various sizes and configurations. Towers are typically classified as either factory-assembled (Figure 11), where the entire tower or a few large components are factory assembled and shipped to the site for installation, or field-erected (Figure 12), where the tower is constructed completely on site. Most factory-assembled towers are of metal construction, usually galvanized steel. Other constructions include stainless steel and fiberglass-reinforced plastic (FRP) towers and components. Field erected towers are predominantly framed of preservative-treated Douglas fir or redwood, with FRP used for special components and casing materials. Environmental concerns about cutting timber and wood preservatives leaching into cooling tower water have led to an increased number of cooling towers having FRP structural framing. Field-erected towers may also be constructed of galvanized steel or stainless steel. Coated metals, primarily steel, are also used for complete towers or components. Concrete and ceramic materials are usually restricted to the largest towers.


6


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MATERIALS OF CONSTRUCTION Materials for cooling tower construction are usually selected to meet the expected water quality and atmospheric conditions. Wood. Wood has been used extensively for all static components except hardware. Redwood and fir predominate, usually with post fabrication pressure treatment of waterborne preservative chemicals, typically chromated copper arsenate (CCA) or acid copper chromate (ACC). These microbicidal chemicals prevent the attack of wood-destructive organisms such as termites or fungi. Metals. Steel with galvanized zinc is used for small and medium sized installations. Hot-dip galvanizing after fabrication is used for larger elements. Hot-dip galvanizing and cadmium and zinc plating are used for hardware. Brasses and bronzes are selected for special hardware, fittings, and tubing material. Stainless steels (principally 302, 304, and 316) are often used for sheet metal, drive shafts, and hardware in exceptionally corrosive atmospheres or to extend unit life. Stainless steel cold-water basins are increasingly popular. Cast iron is a common choice for base castings, fan hubs, motor or gear reduction housings, and piping valve components. Metals coated with polyurethane and PVC is used selectively for special components. Two-part epoxy compounds and epoxy-powdered coatings are also used for key components or entire cooling towers. Plastics. Fiberglass-reinforced plastic (FRP) materials are used for components such as structure, piping, fan cylinders, fan blades, casing, louvers, and structural connecting components. Polypropylene and acrylonitrile butadiene styrene (ABS) are specified for injection-molded components, such as fill bars and flow orifices. PVC is typically used as fill, eliminator, and louver materials. Polyethylene is now used for both hot- and cold-water basins. Reinforced plastic mortar is used in larger piping systems, coupled by neoprene O-ring-gasket bell and spigot joints. Graphite Composites. Graphite composite drive shafts are available for use on cooling tower installations. These shafts offer a strong, corrosion-resistant alternative to steel/stainless steel shafts and are often less expensive, more forgiving of misalignment, and transmit less vibration. Concrete, Masonry, and Tile. Concrete is typically specified for cold-water basins of field-erected cooling towers and is used in piping, casing, and structural systems of the largest towers, primarily in the power and process industries. Special tiles and masonry are used when aesthetic considerations are important.


8

The parts of cooling tower(forced draft): Frame Structucre The eleminators The water distribution system Nozzles Fill Inlet Louvers Fan and motor Basin Strainer Inlet & outlet pipe Over flow pipe Make up Drain


9


10


11


12

Installation and maintance :

1. the hight from the base not less than 1 m. 2. I beam frame under the cooling tower to make the level easy. 3. Gate valve at inlet and outlet pipe. 4. Global valve at out let. 5. strainer at outlet. 6. temperature control at the pipe header outlet. 7. solenoied valve at out let with timer. 8. softener for water treatment. 9. calculate the drift and evaporation not mor than 5%. 10. the location and wind direction. 11. the easy way for mantainance at week, month and seasonal. 12.

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5-Heating / Reheating Coil

6-Empty Module/ Steam Humidifier

7-Supply Fan

8- Built-in HEPA Filter H13

Mixing Box

Panel FilterG4

Supply Fan


Heating / Reheating

Sand traplouvers

HEPA FilterH13

Empty Module

Bag FilterF9 Coil

Steam Humidifier

AHU Year, Humidity Control

Pharmaceutical Clean Room Class “ C “ with terminal HEPA Filters

1-Mixing plenum

2-Cooling Coil

3-Reheating Coil

1-Mixing plenum

2-Heating coil

3-Steam Humidifier


2-Filter G4

3-Bag Filter F9

4-Cooling Coil

5-Heating / Reheating Coil

6-Empty Module/ Steam Humidifier

7-Supply Fan

Mixing Box

Panel FilterG4

Supply Fan


Heating / Reheating

Sand traplouvers

Empty Module

Bag FilterF9 Coil

Steam Humidifier

AHU With Return Air and Fresh Air Units

Pharmaceutical Clean Room Class “ C “ with terminal HEPA Filters

Empty Module

Mixing Box

Steam Humidifier

Panel FilterG4

Panel FilterG4

Sand traplouvers


Precooling CoilSHF= 0.75 : 0.65

Bag FilterF9

Heating Coil

Return Intake

Return FanExhaust

Supply Fan

Swirl DiffuserWith Treminal HEPA FilterH13

Fresh Air FanPreheating Coil Fresh Intake

Fresh Air Duct

Return Air Unit

Supply Air Unit

Fresh Air Unit

Storage of Raw Candy

1-Return Intake

2- Filter G4

3-DX Cooling Coil

4-Reheating Steam Coil

5-Supply Fan

6-Rigid Bag Filter F9

Return Intake

Panel FilterG4 Rigid Bag Filter

F9DX Cooling Coil

Steam Reheating Coil

Supply Fan

Return Intake


F9

After cooler

Supply Fan

1-Return Intake

2- Filter G4

3-Decisscent Wheel

3-After Cooler Coil

5-Supply Fan

6-Rigid Bag Filter F9

All Fresh Outside Air Low Dew Point

Industrial Dehumidification Applications

1-Fresh Intake2- Filter G43-Decisscent Wheel ( 1 )4-After Cooler Coil ( 1 )5-Decisscent Wheel ( 2 )6-After Cooler Coil ( 2 )7-Supply Fan8-Rigid Bag Filter F9

Fresh Intake


F9

After cooler

Supply Fan

After cooler

Decisscent Wheel Decisscent Wheel

AHU EVAPORATIVE Cooling

1-Mixing plenum

2-Chilled Water Spray 1-Mixing plenum

2-Air Washer

1-Mixing plenum

2-Filter G4

3-Air Washer / Chilled Water Spray

4-Eliminator

5-Supply Fan

Mixing Box

Panel FilterAir Washer ( Winter )

Eliminator

Supply FanChilled Water Spray ( Summer )

With ModulatingDampers ( winter )

II- Modules Specifications

Main Specifications

Psychometric Calculations• Supply Air Rate m3/ hr• Outside Air Rate in m3 / hr• Cooling Capacity in tons-ref ( or kW )• Heating Capacity in kW• Steam Humidifier Capacity in kg / hr• Washer Capacity make-up water in lit / min

Cooling Coils

Chilled Water Cooling Coil1. Air Rate

2. Cooling Capacity

3. SHF

4. Air Inlet Condition

5. Chilled Water Inlet Temperature

6. Chilled Water Temperature Rise

DX Cooling Coil1. Air Rate

2. Cooling Capacity

3. SHF

4. Minimum Evaporating Temperature

Heating CoilsHot Water Coil1. Air Rate

2. Heating Capacity


4. Hot Water Inlet Temperature

5. Hot Water Temperature Drop

Electric Heater1. Air Rate

2. Heating Capacity


4. Maximum Surface Temperature

Heat Pump1. Air Rate

2. Heating Capacity


4. Maximum Condensing Temperature

Steam Coil1. Air Rate

2. Heating Capacity


4. Available Steam Pressure

Steam Humidifier

1. Air Rate

2. Heating Capacity


4. Maximum Steam Pressure or Temperature

Air Washer

1. Air Rate

2. Water Consumption


4. Operating Wet bulb

5. Wetting Ratio

6. Humidifying Efficiency

Documents

Equipment Technology forth class By louay A. Mahdi ...uotechnology.edu.iq/dep-MechanicsandEquipment/Lectures and Syllab… · outer cover that acts like fin and the inner plastic