Giáo trình điều khiển hệ thống lạnh

Giáo trình điều khiển hệ thống lạnh

Solenoid Solenoid Coil

Head Pressure Liquid Line Valve Bl oren

Control Filter-Drier

Moisture Liquid Indicator

Regulator —

Accumulator

£ Filter-Drier

Low Pressure Core Control

Air Temp Control And Defrost Control

Temperature Sensing Elements

Temperature Sensing Elements

Fig 13-6 Schematic diagram of simplified thermosutic

control.

Fig 13-27 Bimetal-wpe temperature sensing cÌemenL.

;

_

differenual adjusuments.

Range Adjustment

Glass tube

Contacts

713-11 Space Control Versus

Evaporator Control

Differential Pressure Control

High or Low Pressure Control

Dual-Pressure Controls

adjustment (Changes both point only)

cut-out adjustment (Cut-in factory set)

Fig 13-10 Dual-pressure control (Courtesy of Penn Controls, Inc.)

Pump-Down Cycle

Evaporator Capatity Control

control Closing the solenoid valve in the liquid line o;

segment A renders this porton of the evaporator inopera

tive The capacity reduction obtained is proportional t, the surface area cycled out

Evaporator Capatity Control

Kennard Division, American Air Filter Company, inc.)

Evaporator Capatity Control

Rennar

Evaporator Capatity Control

wary the quanury of air passing over the evaporator As the

damper moves toward the closed posiuon, the resistance

against which the blower must work is increased so that the total quantity of air circulated decreases

so that the bypass damper opens wider as the face damper

is closed off With this arrangement the quantry of air passing over the evaporator can be regulated by allowing more or less air to bypass the evaporator However, regard- less of the position of the dampers, the total quanury of air circulated remains practically the same.

Compressor Capatity Control

Compressor Capatity Control

Typical solenoid vaive (shown energized)

‡ O.D

tubing inlet

—_——- *

Connect to discharge side

of compressor

Wires to electrical

eu ae 2” O.D tubing

Compressor Capatity Control

bypass valve is replaced with a manual or solenoid Stop valve The check valve is required with unloaded starting

to prevent blow-back of the high-pressure gas from the con-

denser.

Compressor Capatity Control

ritor mixes with, and is desuperheated by, the refrigeranc

t the evaporator, thereby eliminating the possibilicy of

Overheating the compressor as a result of excessive super-

heating of the suction vapor In addidon, bypassed hoc

keeps refrigerant velociry in the evaporator relatively high

tnd improves oil return during periods of reduced load- inv Where a refrigeranc distributor is used, it is recom- Mencled that the bypass be connected direcdy-

Compressor Capatity Control

rator upstream of the expansion valve bulb makes it possi- ble to reduce and control suction superheat by overiceding

the evaporacor The expansion valve bulb should be Io-

cated a minimum of 3 ft downstream of the bypass connec- tion A solenoid stop valve is required in pump-—dowm sys- terms to provide positive shucoff of bvoass line

Compressor Capatity Control

Fig 13-19d Hot gas is desuperheated by direct expansion cooling from separate

thermostatic expansion valve This method is ideal for remote systems where the compressor is located some distance away from the evaporator section Where pressure drops are substantial, externally equalized TEVs should be used.

Compressor Capatity Control