AIM: To Find the Logarithmic Mean Temperature Difference (LMTD) effectiveness of Shell and Tube type heat exchanger and to study the heat transfer phenomena in Counter flow arrangements.

APPARATUS:Heat Exchanger, Stop Watch, Measuring Flask, Temperature Sensor with Temperature Indicator.

APPARATUS DESCRIPTION

Heat Exchanger is a device in which heat is transferred from one fluid to another. The necessity for doing this arises in a multitude of industrial applications. Common examples of heat exchangers are the radiator of a car, the condenser at the back of a domestic refrigerator and the steam boiler of a thermal power plant.

Shell and Tube heat exchanger are popular in industries because they occupy less space and offered reasonable temperature drop. The apparatus consist of fabricated SS shell, inside which tubes with baffles on outer side are fitted. This is two pass heat exchanger so that hot water passes to one end of shell through the tubes and returns to another end through remaining tubes. The cold water is admitted at the one end of the shell, which passes over the hot water tubes. Valves are provided to control the flow rate of hot and cold water. Flow rates of hot and water are measured using Rota meters. A magnetic drive pump is used to circulate hot water from a recycle type water tank which is fitted with heaters and digital temperature controller.

The apparatus consists of a tube in tube type concentric tube heat exchanger. The hot fluid is hot water which obtained from an insulated water bath using a magnetic drive pump and it flow through the inner tube while the cold fluid is cold water flowing through the annuals. The hot water flows always in one direction and the flow rate of which is controlled by means of a valve. The cold water can be admitted at one of the end enabling the heat exchanger to run as a parallel flow apparatus or a counter flow apparatus. This is done by valve operations. For flow measurement Rotameters are provided at inlet of cold water and outlet of hot water line. A magnetic drive pump is used to circulate the hot water from a recycled type water tank, which is fitted with heaters and Digital Temperature Controller.

THEORY:

A Transfer type of heat exchanger is one on which both fluids pass simultaneously through the device and heat is transferred through separating walls. In practice most of the heat exchangers used are transfer type ones.

The transfer type exchangers are further classified according to flow arrangement as-Parallel flow in which fluids flow in the same direction.Counter flow in which they flow in opposite direction andCross flow in which they flow at right angles to each other.A simple example of transfer type of heat exchanger in the form of a tube type arrangement in which one of the fluids is flowing through the inner tube and the other through the annulus surroundings it. The heat transfer takes place across the walls of the inner tube.

A heat exchanger in which two fluids exchange heat by coming into direct contact is called a direct contact heat exchanger. Examples of this type are open feed water heaters, desuper heaters and jet condensers. Recuperates are the heat exchangers in which the fluids are separated by a wall. The valve may be a simple plane wall or tube or complex configuration involving fins, baffles and multiple passes of tubes. These units, also called surface heat exchanger are more commonly used because they can be constructed with large heat transfer surfaces in a relatively small volume and are suitable for heating, cooling, evaporating, condensing applications. A periodic flow type of heat exchanger is called a regenerator. This type of heat exchanger, the same space is alternatively occupied by the hot and cold gases between which heat is exchanged. Regenerators find their applications in pre heaters for steam power plants, blast furnaces, oxygen producers etc.

Figure: Counter-Current Flow Arrangement

When large quantities of heat are to be transferred the heat transfer area requirement of heat exchanger also becomes large. In a single pass heat exchanger this requirement can be met either by increasing length of tubes or by decreasing the diameter and increases the nos. of tubes at the same time. Neither these methods is practical because due to limitations of sides, the length of tube can not be increased arbitrarily and large pressure drops could occur which smaller diameter tubes. These difficulties lead us to multi pass arrangement. The fluid flowing through the tubes is called the tube fluid where as the fluid flowing outside the tubes are called the shell fluid. Depending upon the heat transfer area requirement we can have multi tubes and /or shell pass. The flow conditions for shell and tube type heat exchanger are neither parallel flow nor counter flow type. To create turbulence in the shell side fluid and enhance the cross flow velocity of this fluid relative to the tubes, baffles are generally provided. This results in a higher heat transfer coefficient for the outer tube surface.

Heat exchangers are the devices in which heat is transferred from one fluid to another. There are three modes of heat transfer:

1. Conduction2. Convection3. Radiation

In most of the cases the actual transfer of energy as heat is accomplished by more than one these modes of heat transfer. In all cases the total rate of heat transfer may be expressed in terms of a driving force which is decrease in temperature and resistances.

dQ/dT= driving force / Resistances

The heat which is conducted through the body must be frequently be removed by some convection process. A temperature difference is the driving force by which heat is transferred from source (tube side fluid) to receiver (shell side fluid).

LOGARITHMIC MEAN TEMPERATURE DIFFERENCE

For the derivation of temperature difference of two fluids, the following assumptions must be made:

1. The overall coefficient of heat transfer U is constant over the entire length of path.

2. The fluid flow is constant obeying the steady state requirement.

3. The specific is constant over the entire length of path.

4. There are no partial phase changes in the system i.e. vaporization or condensation. The derivation is the application for the sensible heat changes and when vaporization or condensation is isothermal over the whole length of path.

5. The heat loss is negligible.

SPECIFICATIONS

TECHNICAL DETAILS

System : Water to water (Two tube pass, one shell pass type).

Shell : Material stainless steel, 25% cut baffles at 100 mm Distance 4 nos.

Tube : ID 10mm length 500 mm (12 nos.) Material of construction: Stainless steel.

Water flow Measurement : Measuring cylinder and stopwatch with Rota meters (2 nos.) one each for cold and hot fluid.

Hot water tank : Made of stainless steel insulated with ceramic fiber wool.

Hot water circulation : Magnetic pump made of polypropylene to circulate hot water. Maximum working temperature is 85◦ C.

Heaters : 2 kW Nichrome wire heater (2 nos.) Temperature sensors : RTD PT -100 6 nos.

Control panel : Digital temperature controller: 0 -199.9◦ C (for hot water tank). Digital Temperature Indicator: 0 -199.9◦ C, with multi channel Switch.

PROCEDURE

STARTING PROCEDURE FOR SHELL AND TUBE TYPE ARRANGEMENT

1. Clean the apparatus and make the water bath free from dust.

2. Close the entire drain valve provided.

3. Fill the water in the bath and switch on the heater.

4. Adjust the required temperature of hot water using DTC.

5. Adjust the valve. Allow the hot water to recycle in bath through by pass by switching on the magnetic pump.

6. Start the flow through the shell and run the exchanger by opening valves V1 & V2.

7. Adjust the flow rate on cold water side between ranges of 3 to 8 liter per minute.

8. Adjust the flow rate on hot water side between the ranges of 1.5 to 4 liter per minute.

9. Keeping the flow rates same, wait till steady state conditions are reached.

10. Record the temperatures on hot water and cold water side accurately.

FOR DOUBLE PIPE HEAT EXCHANGER ARRANGEMENT CLOS VALVES V1 AND V2 AND OPEN VALVES V3 AND V4.

OBSERVATION TABLE FOR SHELL TYPE ARRANGEMENT.Sr. No.

Flow rate of hot water (mh) in kg/hr.

Hot water inlet temperature (Thi)

T2

Hot water outlet temperature(Tho)

T4

Floe rate of cold water (mc) in kg/hr.

Cold water inlet temperature(Tci)

T1

Cold water outlet temperature(Tco)

T5

CALCULATION

LMTD ( ) can be calculated by following formula:

LMTD

Where ΔTi = Thi-Tco

ΔTo = Tho-Tci

Now heat transfer rate can be calculated as follows:qh= Heat transfer rate for hot water

= m h cph (Thi-Tho) kcal/hr

qc= Heat transfer rate for cold water

= mc cphc (Tco-Tci) kcal/hr

So

Now overall heat transfer coefficient can be found by:

Uri is based upon Ai (π di L)Uro is based upon Ao (π do L)

NOMENCLATUREQ = Heat transfer rate in the direction from point 1 to 2.U = Average overall coefficient of heat transfer (W/m2 ◦ C)A = Area of surface through which heat is transferred (m2)f = Correlation factorΔTlm = Logarithmic mean temperature difference: counter flowThi = Hot water inlet, (T1)Tho = Hot water outlet, (T2)Tci = Cold water inlet, (T3)Tco = Cold water outlet (counter), (T4)Qc = Heat transfer rate for cold streamQh = Heat transfer rate for hot streamUri = Overall heat transfer coefficient based upon Ai (π di L)Uro = Overall heat transfer coefficient based upon Ao (π do L) Ai = Area of surface based on inner diameter of tube. Ao = Area of surface based on outer diameter of tube. di = Inner diameter of tube. do = Outer diameter of tube. L = Length of tube.

Closing procedure:

1. When experiment is over switch OFF heaters.2. Switch OFF pump.3. Switch OFF Power Supply to Panel.4. Stop cooling water supply.5. Drain hot water tank by the drain valve provided.

OBSERVATION & CALCULATION:

DATA:

Di = 0.0095mDo = 0.0127mL = 1.6m

OBSERVATION TABLE:

S.NOMODECOUNTER FLOW FhLPH

Hot water

inT2oC

Hot water out

T3oC

FcLPHCold

water in

T1oC

Cold water out

T6oC

1.

2.

CALCULATIONS:

Find the properties of water (Cph, ρh) at and (Cpc, ρh) at

(As per mode selected) from data book.

Cph = -------------kJ/kgoCCpc = -------------kJ/kgoCρh = -------------kg/m3

ρc = -------------kg/m3

∆T1 =T1-T3, oC (for parallel flow) = -------------------oC

∆T1 = T1-T5, oC (for counter flow) ----------------oC

∆T2 =T2-T4, oC (for parallel flow) = -------------------oC

∆T2 = T2-T3, oC (for counter flow) ----------------oC

NOMENCLATURE:

Ai = Inside heat transfer area, m2

Ao = Outside heat transfer area, m2

Cph = Specific heat of hot fluid at mean temperature, kJ/kgoCCpc = Specific heat of cold fluid at mean temperature, kJ/kgoCDo = Outer diameter of tube, mDi = Inner diameter of tube, mFh = Flow rate of hot water, LPHFc = Flow rate of cold water, LPHL = Length of tube, mMh = Mass flow rate of the hot water, kg/sMc = Mass flow rate of the cold water, kg/sQ = Average heat transfer from the system, WQc = Heat gained by the cold water, WQh = Heat loss by the hot water, WTh = Mean temperature of hot wateroCTc = Mean temperature of cold water, oCT1 = Inlet temperature of the hot water, oCT2 = Outlet temperature of the hot water, oCT3 = Inlet temperature of the cold water, oCT4 = Outlet temperature of the cold water for parallel flow, oCT5 = Outlet temperature of the cold water for counter flow, oC∆Tm = Log mean temperature difference, oCUi = Inside overall heat transfer coefficient, W/m2oCUo = Outside overall heat transfer coefficient, W/m2oCρc = Density of cold water at mean temp, kg/m3

ρh = Density of hot water at mean temp. kg/m3

PRECAUTIONS & MAINTENANCE INSTRUCTIONS:

1. Never run the apparatus if power supply is less than 180volts and above than 230 volts.

2. Never switch ON mains power supply before ensuring that all the ON/OFF switches given on the panel are at OFF position.

3. Operator selectors switch off temperature indicator gently.4. Always keep the apparatus free from dust.

REFERENCES

1. A Text Book of Heat and Mass transfer by Dr. D.S. Kumar.2. A Text Book of Heat and Mass transfer by R. Yadav.3. A Text Book of Fundamentals Engineering Heat and Mass Transfer by R.C.

Sachdeva. 4. Holman, J.P., “Heat Transfer”, 9th ed., Mc Graw Hill, NY, 2008, Page 525-526,

528-531.5. McCabe, Smith, J.C., Harriott, P., “Unit Operations of Chemical

Engineering”, 7th Ed. McGraw Hill, NY, 2005, Page 327-329,331-333.