Brochure

BULLETIN 90-40 / Page 1
SYSTEM CAPACITY CONTROL
On many air conditioning and refrigeration systems it
is desirable to limit the minimum evaporating pres-
sure during periods of low load either to prevent coil
icing or to avoid operating the compressor at a lower
suction pressure than it was designed to operate.
Various methods have been used to achieve this result
— integral cylinder unloading, gas engines with vari-
able speed control, or multiple smaller systems.
Compressor cylinder unloading is used extensively on
larger systems but is too costly on small equipment,
usually 10 hp and below. Cycling the compressor with
a low pressure cutout control is a method often used
but is being re-evaluated for three reasons.
1. On-off control on air conditioning systems is
uncomfortable and does a poor job of humidity
and mold control.
2. Compressor cycling reduces equipment life.
3. In most cases, compressor cycling is not economical
because of pe
ak l
oad demand charges.
One method that offers a practical and economical solu-
tion to the problem, is to bypass a portion of the hot dis-
charge gas directly into the low side. This is done by a
modulating control valve — commonly called a
Discharge Bypass Valve (DBV). This valve, which opens
on a decrease in suction pressure, can be set to automat-
ically maintain a desired minimum evaporating pres-
sure regardless of the decrease in evaporator load.
Sporlan manufactures a complete line of Discharge
Bypass Valves including non-adjustable models, for spe-
cific customer requirements. Contact your Sporlan repre-
sentative for assistance with special needs.
APPLICATION
Sporlan Discharge Bypass Valves provide an economi-
cal method of compressor capacity control in place of
cylinder unloaders or the handling of unloading
requirements below the last step of cylinder unloading.
On air conditioning systems, the minimum allowable
evaporating temperature that will avoid coil icing
depends on evaporator design and the amount of air
passing over the coil. The refrigerant temperature
may be below 32°F, but coil icing will not usually occur
with high air velocities since the external surface tem-
perature of the tube will be above 32°F. For most air
conditioning systems the minimum evaporating tem-
perature is 20°F to 25°F. However, when air velocities
are reduced considerably, the minimum evaporating
temperature should be 26°F to 28°F.
Sporlan Discharge Bypass Valves can be set so they
start to open at an evaporating pressure equivalent to
32°F saturation temperature. Therefore, they would be
at their rated capacity at 26°F evaporating temperature.
On refrigeration systems, discharge bypass valves are
used to prevent the suction pressure from going below
the minimum value recommended by the compressor
manufacturer.
A typical application would be a low temperature com-
pressor designed for operation at a minimum evapo-
rating temperature of -40°F on Refrigerant 22. The
required evaporating temperature at normal load con-
ditions is -30°F. A discharge bypass valve would be
selected which would start to open at the pressure
equivalent to -34°F, and bypass enough hot gas at
-40°F to prevent a further decrease in suction pres-
sure. Valve settings are discussed completely later in
this bulletin.
The discharge bypass valve is applied in a branch l
ine,
off the discharge line, as close to the compressor as
possible. The bypassed vapor can enter the low side at
one of the following locations:
1. Evaporator inlet with distributor.
2. Evaporator inlet without distributor.
3. Suction line.
Each is illustrated and discussed below. While Figures
1, 2, and 3 show a specific type of discharge bypass
valve, all types can be used in place of the one shown.
BYPASSTO EVAPORATOR INLET WITH DISTRIBUTOR
This method of application, illustrated in Figure 1,
provides distinct advantages over the other methods,
especially for unitary or field built-up units where the
high and low side are close coupled.
This method is also applicable on systems with remote
condensing units, especially when the evaporator is
located below the condensing unit, see discussion below.
The primary advantage of this method is that the sys-
tem thermostatic expansion valve will respond to the
increased superheat of the vapor leaving the evaporator
and will provide the liquid required for desuperheating.
Also the evaporator serves as an excellent mixing
chamber for the bypassed hot gas and the
liquid-vapor mixture from the expansion valve. This
ensures a dry vapor reaching the compressor. Oil return
from the evaporator is also improved since the velocity
in the evaporator is kept high by the hot gas.
Sporlan 1650R Series Distributor or ASC – Two refrig-
erant distribution methods are available to introduce
hot gas in this manner:
1. Bypass to Sporlan 1650R series distributor with
an auxiliary side connection.
2. Bypass to Sporlan ASC series Auxiliary Side
Connector.
Method 1 is normally utilized on factory assembled or
unitary systems where hot gas bypass is initially
designed into the system. The 1650R series distribu-
tor allows the hot gas to enter downstream of the dis-
tributor nozzle. Method 2 is applicable on field
built-up systems or on existing systems where the
standard refrigerant distributor is already installed
on the evaporator.
External Equalizer
Evaporator
External
Equalizer
Hot Gas
Solenoid Valve
Compressor
Condenser
Receiver
Catch-All
Side Connection
Distributor or ASC
TEV
See-All
Solenoid Valve
Discharge
Bypass Valve
Figure 1