Refrigeration Flow Control - PowerPoint Presentation

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Refrigeration Flow Control

Instructor Name: (Your Name)

15

CHAPTERSlide2

Learning Objectives

Describe the purpose of refrigerant cycle control devicesExplain the operation of the three-way valve in the cooling cycleExplain the operation of the three-way valve in the heating cycle

Explain the purpose of the pilot solenoidDescribe the purpose of the condenser pressure bypass valveSlide3

Learning Objectives (continued)

Explain the purpose of a check valveExplain the cool/heat/defrost cycle of a three-way valve refrigeration systemExplain the cool/heat/defrost cycle of a solenoid control refrigeration system

Explain the cool/heat/defrost cycle of a four-way reversing valve refrigeration systemSlide4

Three-Way Valve Refrigeration SystemSlide5

Three-Way Valves

Located in discharge line downstream from vibrasorberIn cool operation, directs refrigerant into condenserIn 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 modeSlide6

Three-Way Valve Operation

Operates on principles of spring pressure and differential refrigerant pressureIn cool mode valve is spring holds spool valve against back side of valveSpool 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 compressorHigh-pressure hot gas from compressor flows through bleed passage to both sides of the spool valve equalizing the pressureSlide7

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 systemRefrigerant 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 rightSpool valve blocks flow of refrigerant to condenser and path to evaporatorSlide8

Solenoid ValveSlide9

Three-Way Valves

Three-Way Valve HeatDefrost ModeThree-Way Valve

Cool ModeSlide10

Check Valves

Used by most manufactures of refrigeration equipmentAllows refrigerant flow in one direction but stops in the opposite directionTwo styles used, serviceable and non-serviceable

Serviceable type use a removable cap for access to the seal and springServiceable type usually made of brass

Non-serviceable are inline used to minimize leaksSlide11

Check Valves

Serviceable Check Valve

Non-Serviceable Check ValveSlide12

Refrigerant Flow Three-Way Valve System (Thermo King Units)

Cooling CycleHot gas leaves compressor through vibrasorber the discharge service valveHot gas flows through three-way valve to the condenser

As super heated gas flows through the condenser it changes states to a sub cooled liquidSub cooled liquid flows through the check valve

Liquid is stored in the receiverSlide13

Refrigerant Flow Three-Way Valve System (Thermo King Units)

Refrigerant flows through liquid line to the filter dryerRefrigerant flow through the heat exchanger where it gives up heat to cold suction lineLiquid 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 spaceSlide14

Refrigerant Flow Three-Way Valve System (Thermo King Units)

Cold refrigerant passes through heat exchanger and absorbs more heatRefrigerant then passes through receiver, liquid is separated before suction vibrasorber and service valves and through suction throttling valveLast refrigerant passes through compressor to start cycle overSlide15

Three-Way Valve System Operating in Cool ModeSlide16

Refrigerant Flow Three-Way Valve System (Thermo King Units)

Heat CycleHot gas leaves compressor through vibrasorber the discharge service valveHot gas moves through the discharge vibrasorber and three-way valve

Pilot solenoid is energized, three-way valve is shifted to the heat position stopping flow to condenser and opening flow to drip pan and evaporator

Heat from evaporator is blow into cargo space heating the controlled spaceSlide17

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.Slide18

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 tankThe 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 TXVRefrigerant mixes with hot gas in distributorRefrigerant once trapped in receiver is now used in heat defrost cycle allowing compressor to achieve higher pressures thereby more heatSlide19

Three-Way Valve System Operating in Heat ModeSlide20

Refrigerant Flow Three-Way Valve System (Thermo King Units)

Defrost CycleFlow in defrost cycle is identical to heat cycle

In defrost cycle air is not cycled through the loaded areaClosing damper door traps heat in evaporator compartmentHeat 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 FSlide21

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.Slide22

Operation of Solenoid Control System (Carrier)

Cool ModeIn cool mode, SV3 and SV4 are closed, SV1 and SV2 are openRefrigerant flows from compressor through discharge check valve to condenser, changes state from gas to liquid

Liquid passes through SV1 which is normally openLiquid 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 condenserSlide23

Operation of Solenoid Control System (Carrier)

Exiting the sub-cooler refrigerant pass through the dryer and SV2, which is normally closedRefrigerant 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 spaceQuench 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 compressorSlide24

Solenoid Controlled System Operating in Cool ModeSlide25

Operation of Solenoid Control System (Carrier)

Heating CycleHot 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 modeSV4 will energize and allow refrigerant to pass60 seconds after SV4 is energized if temperature and pressure requirements are met SV3 will be energizedSlide26

Operation of Solenoid Control System (Carrier)

Once SV3 and SV4 open hot gas will flow directly into the evaporatorSV1, 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 SV2SV2 is normally closed and is controlled by HP2 When in heat/defrost modeSV2 is cycled open and closed by HP2 to allow refrigerant to flow to the evaporatorSlide27

Operation of Solenoid Control System (Carrier)

Excess refrigerant trapped in receiver is needed for heat/defrost cycleTeed into TXV feed line is a quench valveQuench 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 spaceSlide28

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 refrigerantBecause of the drop in pressure the refrigerant does not condenseRefrigerant leaves the evaporator and enters suction line, through the suction vibrasorber and into the suction side of compressor

The cycle then repeats itselfSlide29

Solenoid Controlled System Operating in Heat/Defrost ModeSlide30

Four way Valve Operation Cooling Cycle

Refrigerant leaves compressor through service valve and vibrasorber and flows to the four-way valvePilot 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 valveRefrigerant enters the condenser to give up heat to ambient airRefrigerant the flow to the drier, heat exchanger and liquid line check valveSlide31

Four way Valve Operation Cooling Cycle

Refrigerant then enters the TXV, distributor tubes and metered into the evaporatorThe 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 vibrasorberA check valve with orifice prevent refrigerant from entering the drip tray during a cool cycleSlide32

Four-Way Operation Cool CycleSlide33

Four-Way Operation Heat Cycle

In heat cycle four-way reversing valve is shifted to change operation of the evaporator and condenserThe condenser becomes the evaporator absorbing heat from ambient airIn 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 Slide34

Four-Way Operation Heat Cycle

Pilot solenoid (heat) is energized opening passage to suction side of compressorDifferential pressure case valve to shift right opening passage to evaporatorRefrigerant flows through heat exchanger then into the evaporator which is now acting as a condenser

Heat is given up to air passing through evaporator coilSome of the refrigerant passes through the drip pan, the check valve, and orifice, entering condenser as a low-pressure saturated mixtureSlide35

Four-Way Operation Heat Cycle

A check valve in the cool line prevents refrigerant through heat exchanger and dryerLiquid 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 airThe low pressure vapor return to four-way valve, through accumulator and back to suction side of compressorSlide36

Four-Way Operation Heat CycleSlide37

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 refrigerantWhen 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 evaporatorThe 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 cycleSlide38

Summary (continued)

Check valves are used in the refrigeration system to allow refrigerant flow in one direction and stop it in the other directionThe 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 evaporatorSlide39

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 cycleThe 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