Brochure

Parker Hannifin Corporation
Climate & Industrial Controls Group
Cleveland, OH
Flo-Con Pressure Regulators and Valves
336
Catalog CIC-2003-1/US
Hot Gas Bypass
Hot gas bypass is a method of compressor capacity
control that has been utilized in the refrigeration and
air conditioning industry for many years. A significant
number of refrigeration and air conditioning systems
operate at full capacity no more than 20% of their
running time. As a result, many compressors operating
above fifteen horsepower are equipped with cylinder
unloading devices whereby the capacity of the com-
pressor can effectively be reduced. The need for auto-
matic hot gas bypass becomes evident, however, for
those systems having compressors that do not possess
cylinder unloading, or for those systems with unloading
capability, but without or lacking suction pressure
control between the last stage of unloading and 0%
capacity.
A hot gas bypass system is designed to artificially
load the compressor upon a decrease in evaporator
load for one or more of the following reasons:
• To prevent the operation of the compressor at
unacceptably low suction pressures, which could
result in oil pumping, compressor cycling, tem-
perature variation, and eventual compressor or
motor failure.
• To prevent a significant drop in evaporator tem-
perature where constant conditions are required.
• To prevent the frosting of the evaporator coil,
which can cause a loss of capacity due to the
restricted air flow.
Basic hot gas bypass systems are illustrated in Figures
3 and 4.
A portion of the refrigerant discharge gas is bypassed
around the condenser and thermostatic expansion
valve, effectively reducing the flow of refrigerant
available for cooling. In addition to the reduction in
refrigeration effect due to the reduced liquid refrigerant
flow through the condenser and expansion valve, the
heat content of the hot gas bypassed compared to the
suction gas will add load, further reducing the refrigera-
tion effect of the evaporator.
Compressor suction pressure is the principal condition
being controlled with any hot gas bypass system.
Therefore, whenever discharge gas is introduced into
the inlet of the evaporator by means of a hot gas by-
pass regulator, the regulator must be used with an
external equalizer connected to the compressor suction
main, and at a point downstream of any evaporator
pressure regulator which may be used. In this manner,
any pressure drop conditions due to distributors, evapo-
rator circuiting, or valves will not affect the operation of
the hot gas bypass regulator.
The two commercial regulators recommended for hot
gas bypass applications are the Refrigerating Special-
ties A9E, and A8_OE series. The A9E is physically the
smaller of the two valve types, covering port sizes of
3/8" through 1-1/8", and was originally designed spe-
cifically for hot gas bypass applications. The A8 series
covers three different body sizes, the A8AOE, A81OE,
and A82OE, and cover a much larger capacity range,
with port sizes as large as 2 1/8".
Technical Information Hot Gas Bypass