Components of Vapor Compression Refrigeration System

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Ch 3: Components of Vapor
Compression Refrigeration System
Md Rahmatuzzaman Rana
Assistant Professor, Dept of FET, SUST
Objectives
• To let students become familiar with the main components
(evaporator, compressor, condenser, valve) of a refrigeration system
• To investigate the environmental effects of refrigerants emitted in
the atmosphere
Md Rahmatuzzaman Rana
Assistant Professor, Dept of FET, SUST

2. 5/25/2017
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Evaporator
• Air flows through lamels perpendicular to tubes filled with fluid/gas mixture
• Refrigerant absorbs heat and evaporates
• Cooling capacity affected by
- Heat transfer coefficient of evaporator and evaporator surface
- Delta-T, difference between evaporator temperature and cool store
temperature (< 6°C, otherwise dehumidification !)
Md Rahmatuzzaman Rana
Assistant Professor, Dept of FET, SUST
Evaporator
Direct versus indirect expansion
Direct expansion: coils in direct contact with refrigerant
• Cheaper than indirect expansion
• Depending on system, ΔT often relatively large: low relative humidity
Indirect expansion
• Secondary refrigerant (e.g., brine, glycol water mixture) cooled in direct
expansion system
• Secondary refrigerant pumped to heat exchanger in cool room
• Low ΔT possible: high relative humidity
• Expensive
Md Rahmatuzzaman Rana
Assistant Professor, Dept of FET, SUST

3. 5/25/2017
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Evaporator: types
1. Dry-expansion evaporator (a)
– Refrigerant totally vaporised, superheated at exit
– Cheap
2. Flooded-type evaporator (b)
– Recirculation of liquid
– Separation of liquid / vapour, vapour goes to compressor
– No liquid in compressor
=> higher evaporation temperature possible
=> higher RH than direct expansion
– More expensive than dry-expansion evaporator
1. Shell-and-tube evaporator (c)
Md Rahmatuzzaman Rana
Assistant Professor, Dept of FET, SUST
Evaporator: types of evaporator coils
• Bare pipe
– Simple
– Easy to defrost and clean
– Low surface area
• Finned
– Increased surface area
– More expensive
– More difficult to defrost and clean
• Plate
– Intermediate properties
Md Rahmatuzzaman Rana
Assistant Professor, Dept of FET, SUST

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Evaporator
Air flow
• Natural convection: domestic refrigerators, freezers, display cabinets
• Forced convection: fans
Tubing
• Usually copper (high thermal conductivity)
• Not possible with ammonia !
Defrosting
• Lamels covered with ice, no air flow possible anymore
• Defrosting typically on a time basis
• Different defrosting systems:
– Electrical heaters
– Hot ammonia vapour
– Hot glycol
Md Rahmatuzzaman Rana
Assistant Professor, Dept of FET, SUST
Compressor
• Sucks vapor from evaporator
• Compresses vapor
- high temperature (higher than ambient =>increased heat transfer
- high pressure
Compressor Types
1. Positive displacement compressor:
• volume of vapor at evaporator pressure enters and is trapped in the machine
• Trapped volume is reduced while pressure rises
• when pressure reaches the value of the pressure inside the condenser, the compressed
vapour is pushed out into the delivery pipe (discharge line)
• The flow rate is pulsed at each rotation of the machine; the mean value of this pulsed flow
rate is the required mass flow rate required to achieve the desired cooling capacity
• Types: reciprocating, screw, scroll,…
Md Rahmatuzzaman Rana
Assistant Professor, Dept of FET, SUST

5. 5/25/2017
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Compressor
1. Positive displacement compressor: Reciprocating compressor
- C: cylinder filled with low pressure gas
- C-D: piston compresses gas to 15%
- D-A: spring-loaded outlet valve opens, piston drives gas out of cylinder,
cylinder volume decreased to 5%
- A-B: remaining gas expands, outlet valve closes
- B-C: suction valve opens, cylinder filled with low pressure gas
Md Rahmatuzzaman Rana
Assistant Professor, Dept of FET, SUST
Compressor
1. Positive displacement compressor: Screw compressor
E.g., twin screw compressor: two screw rotors, male and female
Trapped gas progressively compressed while moving toward discharge
Md Rahmatuzzaman Rana
Assistant Professor, Dept of FET, SUST

6. 5/25/2017
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Compressor
1. Positive displacement compressor: Scroll compressor
- Rotary machine, compression between two close-fitted spiral scroll members
- one scroll member is fixed, other follows orbital path
Md Rahmatuzzaman Rana
Assistant Professor, Dept of FET, SUST
Compressor
2. Centrifugal compressor
• High speed rotor subjects a steady flow of vapour to centrifugal forces which result in a rise in
pressure.
• Operation
• Vapour enters in (1) at eye of impeller (2), flows through passage (3) to periphery
• Due to high rotating speed of impeller, pressure and velocity raise
• In stationary diffuser(4) kinetic energy of vapour is converted into pressure, decrease of
vapour velocity
• High pressure gas collected in volute (5) and send into discharge line (6)
- Limited pressure ratio, used when large volume flow rates are needed
Md Rahmatuzzaman Rana
Assistant Professor, Dept of FET, SUST

7. 5/25/2017
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Compressor
Compressor capacity affected by
• Piston displacement
• Clearance between piston head and end of cylinder when piston is at top of stroke
• Size of suction and discharge valves
• Revolutions per minute
• Type of refrigerant
• Suction pressure
• Discharge pressure
Control of capacity
• Controlling speed of motor
• Bypassing gas from high pressure side to low pressure side
• Internal bypassing by keeping suction valve open through solenoid (no
savings in power)
Md Rahmatuzzaman Rana
Assistant Professor, Dept of FET, SUST
Condenser
Operation
• Cooling of superheated high pressure vapors from compressor (50 -70°C)
• Condensation to fluid (30 - 40°C)
• Further cooling (undercooling) of refrigerant
Types
1. Water cooled
– Tube-in-tube: water in inner pipe, refrigerant in outer pipe
– Shell-and-coil
– Shell-and-tube
2. Air cooled
- Tube-and-fin
• large heat transfer area
• More complicated construction
- Plate
• Cheaper construction
• Little maintenance
- Artificial movement of air by fan
Md Rahmatuzzaman Rana
Assistant Professor, Dept of FET, SUST

8. 5/25/2017
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Condenser
3. Evaporative condenser
- water is sprayed onto bare tubes carrying the refrigerant.
- Water is recirculated and make-up is necessary.
- As water flows over the tubes, it evaporates and picks up heat from
the refrigerant, causing it to condense.
- The hot humid air formed is eliminated by the air blower and ambient air
admitted.
Md Rahmatuzzaman Rana
Assistant Professor, Dept of FET, SUST
Expansion valve
• Metering device that controls flow of liquid refrigerant to evaporator
Types
1. Manual valves: pressure difference and hence flow rate is fixed
Md Rahmatuzzaman Rana
Assistant Professor, Dept of FET, SUST

9. 5/25/2017
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Expansion valve
2) Capillary tube
• Long copper tube (with a diameter of 0.5-1.5 mm and a length of 1-6m)
• Expansion is caused by the large pressure drop inside the tube
• Mass flow rate of the refrigerant depends on the pressure difference between the condensing
and evaporating sides. It is used in household refrigerators, small commercial systems or
larger systems with very stable working conditions
• Size and the length of the tube must be selected to match the pumping capacity of the
compressor at full load.
• Due to the small size of the tube, the capillary can be easily plugged and a liquid line filter is
needed at the entrance.
3) Automatic valve or pressure control valve
• Regulate flow of refigerant to maintain constant pressure
• IF evaporator pressure very low (large heat load), spring forces valve to
open
Md Rahmatuzzaman Rana
Assistant Professor, Dept of FET, SUST
Expansion Valve
4) Float valves
Low-pressure float valves
– used in systems with flooded evaporator
– valve controls the level of liquid in the evaporator or in a separate
reservoir
– If the cooling load increases, the valve opens wider, allowing more
liquid refrigerant into the coil. Conversely, if less refrigerant boils away,
the valve moves to the closed position
– Often used in large systems using ammonia refrigerant.
Md Rahmatuzzaman Rana
Assistant Professor, Dept of FET, SUST

10. 5/25/2017
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Expansion Valve
4) Float valves
High-pressure float valves
• modulate the refrigerant flow rate to the evaporator by sensing the
liquid level on the high-pressure side of the expansion valve
• As the cooling load increases, more refrigerant is vaporized, pumped
by the compressor and condensed in the condenser.
• Liquid level rises in the high pressure liquid reservoir causing the
expansion valve to open and more refrigerant is fed into the
evaporator.
Md Rahmatuzzaman Rana
Assistant Professor, Dept of FET, SUST
Expansion Valve
5) Thermostatic expansion valve
• Thermostatic bulb (vapour pressure thermometer) is clamped to the
side of the suction pipe to the compressor
• Bulb pressure operates diaphragm which affects orifice
• Relatively large superheating required
6) Electronic expansion valve
- Same as thermostatic expansion valve, but temperature measured
using sensor and valve operated by means of solenoid
Md Rahmatuzzaman Rana
Assistant Professor, Dept of FET, SUST

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Atmospheric emissions of refrigerants
- Poor plant design or operation
- Incorrect or negligent refrigerant handling
- Insufficient plant maintenance
- Accdidental leaks
Ozone-depletion – the Montreal protocol
- Ozone = O3
- Unstable
• Toxic !
• Synthesised through UV (wavelength < 240 nm) in stratosphere (10-50 km)
• Captures UV rays (<320 nm) and prevents them to reach earth
- UV radiation
• Desireable to some extent (vitamine D!)
• Overdose leads to skin cancer (potentially lethal !)
Md Rahmatuzzaman Rana
Assistant Professor, Dept of FET, SUST
• Some refrigerants containing chlorine and bromine (mainly CFCs)
become unstable when irradiated with ultraviolet radiation
• One chlorine radical can trigger 100 to 10,000 ozone breakdown
reactions
• Result: depletion of ozone layer
• Hole in the ozone layer in antarctica, extends to New Zealand and
Australia
Md Rahmatuzzaman Rana
Assistant Professor, Dept of FET, SUST