2. Basic Refrigeration Cycle11
Chapter
Basic Refrigeration Cycle

3. Objectives Draw a diagram of the basic refrigeration cycle.
Describe the condition of the refrigerant in each component.
Explain the purpose of each component of the refrigeration system.
Identify refrigeration system component variations.

4. Basic Refrigeration System
All systems control refrigerant flow
Basic refrigeration cycle
Refrigerant absorbs unwanted heat at one location and releases it at another location
Refrigerants absorb heat by evaporating and release heat through condensing
A thorough understanding of system operation is needed to be a technician

5. Refrigeration System Components

6. Evaporator A heat-exchanging device located in the conditioned space
Performs the main purpose of the system: refrigeration
Liquid refrigerant entering the evaporator leaves as a vapor

7. Superheat Temperature above saturation point
Normal superheat at the evaporator outlet is 10°F (5.5°C)
Provides maximum cooling effect and prevents liquid from leaving the evaporator

8. Evaporator Temperature Difference
Refrigerant inside evaporator tubing is in a low-temperature, low-pressure saturated condition
Evaporator temperature difference (td) must exist for heat to transfer from hot to cold

9. Suction Line Usually made of copper tubing
Connects evaporator outlet to compressor inlet
Insulated to prevent condensation and additional, unwanted superheat
Contains low-temperature, low-pressure superheated vapor

10. Compressor Separates low-pressure from high-pressure side
Removes vapor from evaporator to maintain its low boiling point
Compresses and moves gas through system
Reciprocating type is most common

11. Hot-Gas Discharge Line
Copper tubing that carries discharged hot gas from the compressor to the condenser
Smaller than suction line because the gas has been compressed to a smaller volume and higher pressure
Hottest part of the system outside the compressor

12. Condenser A heat exchanger, designed to release heat
Purpose is to remove heat from the superheated refrigerant vapor
Causing vapor to condense back to a liquid
Can be air-cooled or water-cooled

13. High Pressure in Condenser
Raises the saturation point of the refrigerant so that ambient air can:
Remove superheat to the saturation point (sensible heat, lowering temperature)
Remove enough heat to cause the refrigerant to condense back to a liquid (latent heat)
Remove enough heat to partially subcool the liquid before it leaves the condenser (sensible heat, lowering temperature again)

14. Liquid Line
Copper tubing line connects the condenser outlet to the refrigerant metering device
Should contain only liquid refrigerant (no vapor) subcooled 10°F (5.5°C)
Liquid refrigerant is still under high pressure, and tubing is slightly warm to the touch
Refrigerant inside is a high-temperature, high-pressure, subcooled liquid

15. Refrigerant Metering Device
Controls the flow of liquid refrigerant into the evaporator to ensure that all liquid is boiled off before it enters the suction line
Second division point: between the high- and low-pressure sides of the system
Various types of refrigerant metering devices available for different applications

16. Condition of Refrigerant in Each Component

17. Evaporators
Manufactured in many shapes and sizes to fill specific design or operational needs
Can operate on the principles of conduction or convection
Convection can be natural or forced
Classified into five types
Shell, Shelf, Wall, Plate, Finned-tube with forced convection

18. Evaporator Types Shell-type Shelf-type Wall-type
Tubing run along the inner wall of freezing compartment to form a shell
Shelf-type
Aluminum tubing forms shelves inside a freezer
Wall-type
Used in chest-type domestic freezers
Tubing attached to the surface of inner cabinet

19. Evaporator Types (Cont.)
Plate-Type
Not commonly used
Refrigerant flows through plates
Uses natural convection
Finned tube with forced convection
Most widely used in all applications
Allows much refrigeration with a small evaporator
Fins increase surface area

20. Suction Line Factors determining size of suction line
Refrigerant velocity, pressure, volume, density, and pressure drop
Very long line or having many bends can increase pressure drop
Line should be installed to help all circulating oil return to the compressor

21. Suction Line Insulation
To limit uncontrolled superheating of gas that would reduce compressor efficiency
To eliminate condensation that could cause frost and ice problems

22. Compressor Styles Open Semihermetic Hermetic
Belt-driven or directly driven (using a coupling) by an external electric motor
Semihermetic
Electric motor and compressor sealed within
May be disassembled and opened for service
Hermetic
Motor and compressor sealed in a solid steel body
May not be opened for service

23. Condensers
Must remove heat from refrigerant at same rate heat is being absorbed
For proper condenser operation, compressor must increase refrigerant pressure to saturation temperature
30°F to 35°F (17°F to 19°C) higher than ambient air entering the condenser (for air-cooled)
20°F (11°C) higher than temperature of the water exiting the condenser (water-cooled)

24. Condenser Cooling Stages
First, the superheat of the gas is removed, cooling the gas down to the saturation point
Sensible heat removed
Second, the gas is condensed to a liquid
Latent heat removed
Third, the liquid is subcooled below its saturation point

25. Domestic Condensers Finned-tube, forced convection Wire static
Most commonly used condenser
Fan creates airflow for convection
Wire static
Natural convection with wires attached to tubing
Wall static
Natural convection with a box inside a box, having condenser tubing mounted along outer wall

26. Commercial Air-Cooled Condensers
Heat is removed through airflow
Most common type is forced convection
Vary greatly in size, style, and capacity
Some mounted vertically, some horizontally

27. Commercial Water-Cooled Condenser
Heat is removed through conduction into flowing water
Very efficient at cooling refrigerant
Water-related problems
Types available
Tube-in-a-tube
Tube-in-a-shell
Coil-in-a-shell

28. Cooling Tower
Lowers water temperature for reuse in a water-cooled condenser
Usually located outdoors where ambient air can cool the water

29. Water-Regulating Valve
Controls amount of water traveling through the condenser
A bellows and diaphragm assembly connects by capillary tube to the high-pressure side of the compressor
As head pressure increases, the valve opens more to permit more waterflow
Spring tension is adjustable and permits control of head pressure