User Guide

ManualsBrandsSporlan ManualsBF/EBF/SBF SeriesSBFV-AAA-C 3/8" x 1/2" ODF Thermal Expansion Valve w/ 30" Capillary (1/8 to 1/3 Ton)

Hot Gas Bypass and Desuperheating TEVs

Systems which are required to operate at load conditions

below the unloading capabilities of their compressors pres-

ent an additional design problem. To balance the system

under these conditions, bypassing a controlled amount of

hot gas to the suction side of the system provides a practical

solution. Bypassing hot gas is accomplished with a modu-

lating control valve known as a discharge bypass valve.

Sporlan manufactures a complete line of these valves. For

details, refer to Bulletin 90-40.

For close coupled systems, the preferred method of hot gas

bypass is bypassing to the inlet of the evaporator. This

method has three advantages: (1) the TEV will respond to

the increased superheat of the vapor leaving the evapora-

tor and will provide the liquid required for desuperheating;

(2) the evaporator serves as an excellent mixing chamber

for the bypassed hot gas and the liquid vapor mixture from

the TEV; and (3) oil return from the evaporator is improved

since the refrigerant velocity in the evaporator is kept high

by the hot gas.

For multi-evaporator or remote systems, bypassing hot gas

directly into the suction line in the manner illustrated in

Figure 11 may be necessary. In addition to the discharge

bypass valve, an auxiliary TEV known as a desuperheating

TEV is required to supply the necessary liquid refrigerant to

cool the discharge gas entering the suction line. Compressor

manufacturers generally rate their air conditioning compres-

sors for a 65°F return gas temperature, and this temperature

is usually appropriate for selecting desuperheating TEVs.

Many refrigeration and low temperature compressors, how-

ever, require lower suction gas temperatures to prevent dis-

charge gas temperatures from rising too high and damaging

compressor parts and carbonizing oil. Consult the compressor

manufacturer if the maximum permissible suction gas tem-

perature for a compressor is not known.

Sporlan has developed special desuperheating thermostatic

charges. See Table 5 below. Each charge will allow the desu-

perheating TEV to control the listed suction gas superheat.

For suction gas temperatures that require superheats other

than those listed, contact the Sporlan Division of Parker or

the compressor manufacturer for assistance.

Sizing a desuperheating valve involves determining the

amount of refrigerant liquid necessary to reduce the suc-

tion gas temperature to the proper level. For hot gas bypass

applications, a desuperheating valve can be properly sized

from the selection procedure provided in Bulletin 90-40.

An externally equalized TEV is recommended for most desu-

perheating applications. If the piping of the desuperheating

TEV is close coupled, an internally equalized valve may be

used. Figure 11 illustrates the use of an externally equal-

ized desuperheating TEV. Refer to the section, Equalization

Method on Page 5 for further information on this subject.

When piping the discharge bypass valve and the desuper-

heating TEV, remember that good mixing of the discharge

gas and liquid must be obtained before the mixture reaches

the sensing bulb of the desuperheating TEV. Improper mix-

ing may produce unstable system operation causing the

desuperheating TEV to hunt. Proper mixing can be accom-

plished in two ways: (1) install a suction line accumulator

downstream of both valve outlet connections with the desu-

perheating TEV bulb downstream of the accumulator; or (2)

mix the liquid vapor mixture from the desuperheating TEV

and the hot gas from the bypass valve together before con-

necting a common line to the suction line. The latter method

is illustrated in Figure 11.

Off-Cycle Pressure Equalization

Certain applications utilizing low starting torque single

phase compressor motors (e.g., a permanent split capacitor

motor) require some means of pressure equalization during

system offcycle. Pressure equalization is necessary since low

starting torque compressors are not capable of restarting

against a large pressure differential. Typical applications

requiring pressure equalization are small air conditioning

and heat pump systems which frequently cycle on and off in

response to a thermostat.

Permanent Bleed Port — Any Sporlan thermostatic

expansion valve may be ordered with a bleed port. Standard

bleed port sizes are: 5%, 10%, 15%, 20%, 30%, and 40%.

Bleed ports are designated by the percentage they increase

nominal valve capacity at 40°F evaporator temperature.

For example, a 2 ton TEV with a 30% bleed will have the

capacity of: 2 x 1.3 = 2.6 tons. Refer to Page 17 for Ordering

instructions. Please contact Sporlan for assistance in select-

ing appropriate bleed port sizes.

The subject of pressure equalization during system off-

cycle should not be confused with the external equalizer of

the TEV. System pressure equalization is accomplished by

allowing a certain amount of refrigerant to leak through a

machined notch or hole in the valve seat during system off-

cycle. The external equalizer of the TEV, however, simply

allows the valve to sense evaporator pressure. The external

equalizer does not provide pressure equalization during

system off-cycle.

Page 12 / BULLETIN 10-9

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* For suction gas temperatures that require superheats other than those listed

above, contact the Sporlan Division of Parker or the compressor manufacturer

for assistance.

External Equalizer

Evaporator

Compressor

See•All

Solenoid

Valve

TEV

Catch-All

Condenser

Receiver

External Equalizer

Desuperheating

TEV

Hot Gas

Solenoid Valve

Discharge

Bypass

Valve

Figure 11

Table 5