HVAC systemHVAC stands for heating, ventilation and air conditioning, HVAC is particularly important in the design of medium to large industrial and office buildings such as skyscrapers and in hospitals where safe and healthy building conditions are regulated with temperature and humidity, as well as "fresh air" from outdoors.

HVAC system operation description:

Chilled water cycle

In operation a liquid water to be chilled flows through the evaporator, where boiling refrigerant absorbs heat from liquid, the chilled liquid is then piped to air handle units through primary and secondary pumps, absorbing the heat from the air the warmed liquid is then returned to the chiller to complete the chilled liquid circuit.

FIG. 1-typical chilled water cycle

Condenser water cycle

Condenser water is supplied to the chiller from an external source like cooling tower which condensed the hot vapour refrigerant and pumped to cooling tower for rejection of heat and then pumped to the condenser again after mixed with makeup water

The chiller requires only that the minimum condenser water temperature be no lower than the value determined by this formula

min ECWT=LCWT - C range + 17 F°

ECWT: Entering condenser water temperature

LCWT: leaving chilled water temperature

C range :condensing water temperature range at the given load condition

C range = ( load percent10 ) * 1.5 in F°

FIG. 2-typical condenser water cycle

Refrigerant cycle

The refrigerant vapour, which is produced by the boiling action in the evaporator ,flows to the compressor where rotating impeller increases its pressure, temperature and discharges it into condenser, water flowing through the condenser tubes absorb heat from the refrigerant vapour causing it to condense , the condensed refrigerant drains from condenser into liquid return line where the expansion orifice which decrease the pressure of liquid refrigerant and meters the flow of liquid refrigerant to the evaporator to complete the cycle.

FIG. 3-– REFRIGERANT FLOW-THRU CHILLER

HVAC system mainly components in Children's Cancer Hospital Egypt 57357:

3 chillers each one capacity 700 TR 22 air handling unit 3 cooling tower 202 Fan coil unit served patient room and shielded rooms

1. Chillers

Its machines that chill water to circulate to airside components throughout the building

FIG. 4 -chiller front view

FIG. 5-chiller rear view

Mainly consist of:

1. Evaporator

Construction description

Its One-shell pass and two-tube passes heat exchanger flooded type heat exchanger. A distributor trough provides uniform distribution of refrigerant over the entire shell length. Stainless steel mesh eliminators or suction baffles are located above the tube bundle to prevent liquid refrigerant carryover into the compressor. A 2" liquid level sight glass is located on the side of the shell to aid in determining proper refrigerant charge. The evaporator shell contains dual refrigerant relief valves.

2. Condenser

Construction description

Its One-shell pass and two-tube passes heat exchanger with a discharge gas baffle to prevent direct high velocity impingement on the tubes. A separate subcooler is located in the condenser to enhance performance. Dual refrigerant relief valves are located on condenser shells with optional isolation refrigerant isolation valves.

Design loadevaporator Condenser

Passes 2 2Design working pressure

300 300

Fouling factor 0.0005 0.001Pressure drop 10.6 8.8Nozzle arrangement in

2 17

Nozzle arrangement out

3 18

Return temp 57 85Leaving temp 42 100GPM 1120 1325Tube 181 260

3. Compressor

Its single stage centrifugal type powered by electric motor and fitted with pre rotation vanes which control the capacity and maintain a constant leaving chilled liquid temperature

4. Compressor lubrication system

The chiller lubrication system consists of oil pump, oil heater, oilfilter; oil cooler and all interconnecting oil piping and passages, there are main points within the motor and compressor which must be supplied with forced lubrication as follows:

1. Compressor Drive Shaft (Low Speed)a. Shaft seal.b. Front and rear journal bearings – one on each side of driving gear.c. Low speed thrust bearing (forward and reverse).2. Compressor Driven Shaft (High Speed)a. Forward and reverse high speed thrust Bearing.b. Two journal bearings.3. Speed Increasing Gearsa. Meshing surfaces of drive and pinion gear teeth.

Components of compressor lubrication system:

OIL PUMPFor normal operation, the oil pump should operate at all times during chiller operation.On shutdown of the system for any reason, the oil pump operates and continues to run for 150 seconds. The system cannot restart during that time interval.

OIL HEATERDuring long idle periods, the oil in the compressor oil reservoir tends to absorb as much refrigerant as it can hold, depending upon the temperature of the oil and the pressure in the reservoir. As the oil temperature is lowered, the amount of refrigerant absorbed will beIncreased. If the quantity of refrigerant in the oil becomes excessive, violent oil foaming will result as the pressure within the system is lowered on starting. This foaming is caused by refrigerant boiling out of the oil as the pressure is lowered. If this foam reaches the oil pump suction, the bearing oil pressure will fluctuate with possible temporary loss of lubrication, causing the oil pressure safety cutout to actuate and stop the system.

Oil cooler It’s used to cool the oil when it is over heated and to achieve the optimum viscosity range of oil which decreases the friction to the minimum value.

Oil filterUsed to get rid from the foreign solid particle like aluminum particle which may be exits due the relative motion or the friction between the compressor and its casing

5. MotorResponsible for drive the compressor and control of pre rotation vane position we have to types1- solid state starter

Is a reduced voltage starter that controls and maintains a constant current flow to motor during stat up

Disadvantage: Advantagenormal efficiency lower costCan’t be used in emergency will cause a failure to the emergency generator due to the high starting current

2 -variable speed drive

It’s designed to vary the compressor motor speed and pre rotation vane position by controlling the frequency and voltage of electrical power to motor. The control logic automatically adjust motor speed and compressor pre rotation vane position for maximum part load efficiency by analyzing information fed to it by sensor located throughout the chiller

Advantage Disadvantage

Can be used in emergency due to gradually higher cost

Increase in the current at start up

Higher efficiency

2. Air handler components and fan coil unit

1) Blower/fan

Air handlers typically employ a large squirrel cage blower driven by an AC induction motor. Flow rate may also be controlled by inlet vanes or outlet dampers on the fan. Some residential air handlers (central 'furnaces' or 'air conditioners')

Multiple blowers may be present in large commercial air handling units, typically placed at the end of the AHU and the beginning of the supply ductwork (therefore also called "supply fans"). They are often augmented by fans in the return air duct ("return fans") pushing the air into the AHU. This also can be useful in case of fire by operating the return fan to draw the smoke and open the exh.damper 100% and when the fire was putdown operate normal to provided fresh air

2) Heating and/or cooling elements

Air handlers need to provide both heating and cooling to change the supply air temperature depending on the location and the application.

air handling units contain coils that circulate hot water for heating, and chilled water for cooling. Coils are typically manufactured from copper for the tubes, with copper or aluminium fins to aid

heat transfer. Cooling coils will also employ eliminator plates to remove and drain condensate. The hot water is provided by a boiler, and the chilled water is provided by a chiller.

3) Mixing chamber

In order to maintain indoor air quality, air handlers commonly have provisions to allow the introduction of outside air into, and the exhausting of air from the building. In temperate climates, mixing the right amount of cooler outside air with warmer return air can be used to approach the desired supply air temperature. A mixing chamber is therefore used which has dampers controlling the ratio between the return, outside, and exhaust air.By that I can decreses the load on the chillers or boilers and saving power and increase efficiency

4) Control and safety

DPS (differential pressure switch)

FIG. 6-DPS

Differential pressure switches used for monitoring filter, fan and heat exchange coils in air handling systems.

We placed DPS( differential pressure switch) in three in the A.H.U

Heat exchange coil

to monitor the flow of air on the water tubes and fins any pressure difference send signal to BMP control system and it indicate that the contamination of dust or fouling on tubes which obstacle the flow of air

Filters

Gives an indication about the condition of the filter.If the filter is Plugged measure high pressure difference after and before the filter and Maintenance team must disassembly the filter and clean it

Blower/fan

To check that the blower work safely and if problems in the electrical motor or speed transmission belts will be observed through difference in pressure between the intake side and discharge side