Refrigeration Flow Control

1. CHAPTER Refrigeration Flow Control 15 Instructor Name:(Your Name)

2. Learning Objectives • Describe the purpose of refrigerant cycle control devices • Explain the operation of the three-way valve in the cooling cycle • Explain the operation of the three-way valve in the heating cycle • Explain the purpose of the pilot solenoid • Describe the purpose of the condenser pressure bypass valve

3. Learning Objectives (continued) • Explain the purpose of a check valve • Explain the cool/heat/defrost cycle of a three-way valve refrigeration system • Explain the cool/heat/defrost cycle of a solenoid control refrigeration system • Explain the cool/heat/defrost cycle of a four-way reversing valve refrigeration system

4. Three-Way Valve Refrigeration System

5. Three-Way Valves • Located in discharge line downstream from vibrasorber • In cool operation, directs refrigerant into condenser • In heat/defrost mode hot gas is directed to the evaporator • Three-way valve is controlled by the pilot solenoid, an electrically operated valve used only during the heat or defrost mode

6. Three-Way Valve Operation • Operates on principles of spring pressure and differential refrigerant pressure • In cool mode valve is spring holds spool valve against back side of valve • Spool valve seat blocks flow to the evaporator leaving the condenser side open • Pilot solenoid is de-energized blocking three-way valve end cap from suction side of compressor • High-pressure hot gas from compressor flows through bleed passage to both sides of the spool valve equalizing the pressure

7. Three-Way Valve Operation (continued) • In heat/defrost mode pilot solenoid is energized and opens right side of spool valve piston to suction side of system • Refrigerant from the bleed passage is drawn into the suction side of the compressor • Discharge pressure on left side of spool valve piston overcomes the spring pressure and suction pressure on right side of spool valve piston, piston shifts to the right • Spool valve blocks flow of refrigerant to condenser and path to evaporator

8. Solenoid Valve

9. Three-Way Valves Three-Way Valve Cool Mode Three-Way Valve Heat Defrost Mode

10. Check Valves • Used by most manufactures of refrigeration equipment • Allows refrigerant flow in one direction but stops in the opposite direction • Two styles used, serviceable and non-serviceable • Serviceable type use a removable cap for access to the seal and spring • Serviceable type usually made of brass • Non-serviceable are inline used to minimize leaks

11. Check Valves Serviceable Check Valve Non-Serviceable Check Valve

12. Refrigerant Flow Three-Way Valve System (Thermo King Units) Cooling Cycle • Hot gas leaves compressor through vibrasorber the discharge service valve • Hot gas flows through three-way valve to the condenser • As super heated gas flows through the condenser it changes states to a sub cooled liquid • Sub cooled liquid flows through the check valve • Liquid is stored in the receiver

13. Refrigerant Flow Three-Way Valve System (Thermo King Units) • Refrigerant flows through liquid line to the filter dryer • Refrigerant flow through the heat exchanger where it gives up heat to cold suction line • Liquid then passes through the TXV where is metered through the distributor tubes to the evaporator • The pressure drop across the TXV causes the refrigerant to boil and absorb heat from the controlled space

14. Refrigerant Flow Three-Way Valve System (Thermo King Units) • Cold refrigerant passes through heat exchanger and absorbs more heat • Refrigerant then passes through receiver, liquid is separated before suction vibrasorber and service valves and through suction throttling valve • Last refrigerant passes through compressor to start cycle over

15. Three-Way Valve System Operating in Cool Mode

16. Refrigerant Flow Three-Way Valve System (Thermo King Units) Heat Cycle • Hot gas leaves compressor through vibrasorber the discharge service valve • Hot gas moves through the discharge vibrasorber and three-way valve • Pilot solenoid is energized, three-way valve is shifted to the heat positionstopping flow to condenser and opening flow to drip pan and evaporator • Heat from evaporator is blow into cargo space heating the controlled space

17. Refrigerant Flow Three-Way Valve System (Thermo King Units) • If in the defrost mode a damper door solenoid is energized blocking off air flow to controlled space. Ice melts and coil dries but controlled space stays cool. • Cool vapor along with some liquid pass through to the heat exchanger to accumulator. Liquid is separated and vapor returns to compressor. • The accumulator may be heated to aid in vaporization liquid refrigerant.

18. Refrigerant Flow Three-Way Valve System (Thermo King Units) • High pressure refrigerant is teed off at the drip pan to the bass pass check valve at the receiver tank • The condenser check valve is held closed blocking the condenser, refrigerant in the receiver can only exit through the tank outlet valve • Liquid refrigerate passes through the drier and passes through a notch in seat or internal orifice of TXV • Refrigerant mixes with hot gas in distributor • Refrigerant once trapped in receiver is now used in heat defrost cycle allowing compressor to achieve higher pressures thereby more heat

19. Three-Way Valve System Operating in Heat Mode

20. Refrigerant Flow Three-Way Valve System (Thermo King Units) Defrost Cycle • Flow in defrost cycle is identical to heat cycle • In defrost cycle air is not cycled through the loaded area • Closing damper door traps heat in evaporator compartment • Heat builds up melting ice, water drips into defrost pans to a pair of drain tubes • When trailer box temperatures become very low, melted water can freeze in pan • Defrost pan heaters are coils that hot gas pass through to defrost the drip pans • Most units will not defrost until evaporator reaches 45 degrees F and will terminate at 55 degrees F

21. NOTE It is harmful to the cargo to have warm air circulated through temperature sensitive products that are required to keep frozen or refrigerated. For this reason, if a problem is detected with the defroster door (not closing), it must be replaced immediately because the unit will not come out of heat/defrost cycle until the whole trailer has warmed up far above the set point.

22. Operation of Solenoid Control System (Carrier) Cool Mode • In cool mode, SV3 and SV4 are closed, SV1 and SV2 are open • Refrigerant flows from compressor through discharge check valve to condenser, changes state from gas to liquid • Liquid passes through SV1 which is normally open • Liquid then flow through the receiver where excess liquid is stored • Liquid then flows out king valve and into the sub-cooler which is another portion of the condenser

23. Operation of Solenoid Control System (Carrier) • Exiting the sub-cooler refrigerant pass through the dryer and SV2, which is normally closed • Refrigerant flows through the TXV which meters flow to controls a constant superheat at evaporator outlet • The pressure drop caused by the TXV causes the refrigerant to boil in the evaporator and absorb heat from the controlled space • Quench valve, a small TXV senses the compressor discharge, if unsafe pressures are sensed it will allow small amounts of liquid refrigerant into suction line, it boils off in pressure drop and cools the compressor

24. Solenoid Controlled System Operating in Cool Mode

25. Operation of Solenoid Control System (Carrier) Heating Cycle • Hot vapor leaves compressor through discharge service valves, discharge vibrasorber through discharge check valve • Refrigerant flows to SV3 and SV4, these valves are initially closed when placed in heat mode • SV4 will energize and allow refrigerant to pass • 60 seconds after SV4 is energized if temperature and pressure requirements are met SV3 will be energized

26. Operation of Solenoid Control System (Carrier) • Once SV3 and SV4 open hot gas will flow directly into the evaporator • SV1, normally open, will energize and close stopping refrigerant flow through the condenser • Hot gas flows through the bypass check valve filling the receiver, subcooler, filter dryer and is stopped at SV2 • SV2 is normally closed and is controlled by HP2 When in heat/defrost mode • SV2 is cycled open and closed by HP2 to allow refrigerant to flow to the evaporator

27. Operation of Solenoid Control System (Carrier) • Excess refrigerant trapped in receiver is needed for heat/defrost cycle • Teed into TXV feed line is a quench valve • Quench valve, a small TXV senses the compressor discharge, if unsafe pressures are sensed it will allow small amounts of liquid refrigerant into suction line, it boils off in pressure drop and cools the compressor • The superheated refrigerant that get gets past SV3 and SV4 enter the evaporator and transfer heat to the evaporator coils to heat the cargo space

28. Operation of Solenoid Control System (Carrier) • The transfer of heat from refrigerant and suction of compressor cause the pressure to drop with a corresponding drop in the boiling temperature of the refrigerant • Because of the drop in pressure the refrigerant does not condense • Refrigerant leaves the evaporator and enters suction line, through the suction vibrasorber and into the suction side of compressor • The cycle then repeats itself

29. Solenoid Controlled System Operating in Heat/Defrost Mode

30. Four way Valve Operation Cooling Cycle • Refrigerant leaves compressor through service valve and vibrasorber and flows to the four-way valve • Pilot solenoid (cool) is energized opening a passage o the suction side • Differential pressure causes valve to shift left opening passage to the condenser side of four way valve • Refrigerant enters the condenser to give up heat to ambient air • Refrigerant the flow to the drier, heat exchanger and liquid line check valve

31. Four way Valve Operation Cooling Cycle • Refrigerant then enters the TXV, distributor tubes and metered into the evaporator • The pressure causes refrigerate to boil and absorb heat in the cargo space • Refrigerant passes through heat exchanger, four-way valve then enters the accumulator • Refrigerant exits the accumulator and return to the compressor through the suction line and suction vibrasorber • A check valve with orifice prevent refrigerant from entering the drip tray during a cool cycle

32. Four-Way Operation Cool Cycle

33. Four-Way Operation Heat Cycle • In heat cycle four-way reversing valve is shifted to change operation of the evaporator and condenser • The condenser becomes the evaporator absorbing heat from ambient air • In cold environments shutters may have to be held shut in order to boil refrigerant • Super-heated refrigerant leaves compressor through discharge service valve and discharge vibrasorber to four-way

34. Four-Way Operation Heat Cycle • Pilot solenoid (heat) is energized opening passage to suction side of compressor • Differential pressure case valve to shift right opening passage to evaporator • Refrigerant flows through heat exchanger then into the evaporator which is now acting as a condenser • Heat is given up to air passing through evaporator coil • Some of the refrigerant passes through the drip pan, the check valve, and orifice, entering condenser as a low-pressure saturated mixture

35. Four-Way Operation Heat Cycle • A check valve in the cool line prevents refrigerant through heat exchanger and dryer • Liquid refrigerant passes through expansion valve (heat) at inlet of condenser, through the check valve, then enters the condenser • The refrigerant changes state by absorbing heat from ambient air • The low pressure vapor return to four-way valve, through accumulator and back to suction side of compressor

36. Four-Way Operation Heat Cycle

37. Summary • Refrigerant control valves are used to control the refrigerant units mode of operation from cool to heat/defrost mode by directing the flow of superheated refrigerant • When the three-way valve is in the cool cycle, it directs the flow of refrigerant into the condenser • When the three-way valve is in the heat cycle, it directs the flow of refrigerant into the evaporator • The function of the condenser pressure bypass valve is to ensure condenser pressure does not exceed the discharge pressure, enabling the three-way valve to shift from heat to cool cycle

38. Summary (continued) • Check valves are used in the refrigeration system to allow refrigerant flow in one direction and stop it in the other direction • The solenoid control system uses three or four liquid and vapor line solenoids to control the flow of refrigerant for heat, cool, and defrost cycles • The four-way valves completely change the direction of refrigerant flow in the heat/defrost mode by turning the evaporator into the condenser and the condenser into the evaporator

39. Summary (continued) • The four-way valve system requires two TXV valves due to the changing of the normal condenser to an evaporator during heat/defrost cycle • The flow of the refrigerant in the defrost cycle is identical to that of the unit’s heating cycle, regardless of manufacturer. All units also use some means of stopping the air flow through the cargo space in the defrost mode