[PPT]Refrigeration Flow Control - Higher Ed eBooks & Digital ... · Web viewLearning Objectives (continued) Explain the purpose of a check valve Explain the cool/heat/defrost cycle

Copyright © 2014 Delmar, Cengage Learning

Refrigeration Flow Control

Instructor Name: (Your Name)

15CHAPTER


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


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


Three-Way Valve Refrigeration System


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


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


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


Solenoid Valve


Three-Way Valves

Three-Way Valve HeatDefrost Mode

Three-Way ValveCool Mode


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


Check Valves

Serviceable Check Valve Non-Serviceable Check Valve


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



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



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


Three-Way Valve System Operating in Cool Mode



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 position stopping flow to condenser and opening flow to drip pan and evaporator

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



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.



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


Three-Way Valve System Operating in Heat Mode



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


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.


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


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


Solenoid Controlled System Operating in Cool Mode



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



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



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



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


Solenoid Controlled System Operating in Heat/Defrost Mode


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


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


Four-Way Operation Cool Cycle


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


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


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


Four-Way Operation Heat Cycle


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


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


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

Documents

[PPT]Refrigeration Flow Control - Higher Ed eBooks & Digital ... · Web viewLearning Objectives (continued) Explain the purpose of a check valve Explain the cool/heat/defrost cycle