Brochure

PAGE 6 / Bulletin 100-60
All step motor valves, however, will remain at their current
position if power is removed during operation. In some criti-
cal systems an additional hot gas solenoid may be desirable
to ensure that no hot gas is bypassed during system or power
failures. When used, the hot gas solenoid valve may be wired
in series with a bi-metal thermostat fastened to the discharge
line close to the compressor. This causes the solenoid valve
to close if the discharge line temperature becomes exces-
sive. Complete selection information is given in the Sporlan
Solenoid Bulletin 30-10.
THE NEED FOR HEAD PRESSURE
CONTROL
A discharge bypass valve can be applied on any system
that experiences undesirable compressor cycling during
periods of low load or for direct temperature control of
any evaporator. Normally, when hot gas bypass is used for
capacity control during periods of low load, outdoor ambi-
ent may be below 70°F. Therefore, all air cooled systems
that utilize hot gas bypass for capacity control should have
some type of head pressure control to maintain satisfactory
performance. Sporlan Bulletin 90-30 Head Pressure Control
Valves has complete selection and application information.
SPECIFICATIONS
Sporlan Electric Hot Gas Bypass valves utilize many of the
proven construction features of Sporlan step motor electric
expansion valves and evaporator control valves. They are
constructed of the finest materials — those best suited for
the specific purpose intended for each valve component. This
ensures long life and dependable service.
SELECTION
The selection of a hot gas bypass valve and the necessary
companion devices is simplified if complete system infor-
mation is available. This will result in the most economical
selection because the components will match the system
requirements. Besides the discharge bypass valve, a specific
application may require a hot gas solenoid valve, an auxiliary
side connection distributor or ASC adapter, and a desuper-
heating TEV with a companion liquid line solenoid valve.
Once the type of application (review Application Section
pages 3-6) is determined, the necessary valves can be selected
from the information discussed in this section.
The selection of a Sporlan Electric Hot Gas Bypass valve
involves five basic items:
1. Refrigerant – valve capacities may vary consider-
ably for the different refrigerants.
2. Desired outlet fluid temperature – depending
on the system, this value must be set to prevent coil
icing and/or compressor short cycling. For example,
this may be 32-34°F for a water chiller; 26-28°F for
a normal air conditioning system; and, the freezing
temperature of the specific product for a refrigeration
system.
3. Compressor capacity (tons) at minimum allow-
able evaporating temperature – consult compres-
sor capacity ratings for this value.
4. Minimum evaporator load (tons) at which the sys-
tem is to be operated – most systems are not required
to operate down to zero load but this value will depend
on the type of system. For example, most air condition-
ing systems only operate down to 15-25% of full load.
However, air conditioning systems for data processing
and “white” rooms, and most refrigeration systems may
be required to bypass to zero load conditions.
5. Condensing temperature when minimum load
exists – since the capacity ratings of the bypass
valves are a function of condensing temperature,
it is vital that proper head pressure is maintained,
especially during low load operation. As the capacity
table indicates, a condensing temperature of 80°F is
considered the minimum allowable for satisfactory
systems operation. See Bulletin 90-30 for information
on Sporlan’s Head Pressure Control Valves.
The hot gas bypass valve must be selected to handle the dif-
ference between items 3 and 4 above. If the minimum evapo-
rator load (item 4) is zero, the hot gas bypass requirement is
simply the compressor capacity at the minimum allowable
evaporating temperature (item 3). The following discussion
on Capacity Ratings and the example show how these fac-
tors affect a selection on a typical air conditioning system.
Capacity Ratings - As the Hot Gas Bypass Valve Capacity
Table indicates, valve ratings are dependent on the evaporat-
ing and condensing temperature at the reduced load condition
and the refrigerant used. Therefore, once this information and
the hot gas bypass requirement in tons is determined, a hot
gas bypass valve can be selected.
Example – Select a hot gas bypass valve for a 30 ton,
Refrigerant 22, air conditioning system with 67% cylinder
unloading (4 of 6 cylinders unloaded). Normal operating con-
ditions are 45°F evaporating temperature and 120°F condens-
ing temperature with a minimum condensing temperature of
80°F due to head pressure control. When the evaporator load
drops below the last step of cylinder unloading, it is necessary
to keep the system on the line to maintain proper space tem-
peratures and avoid frosting of the coil. From the compressor
manufacturer’s capacity table, the compressor capacity in
tons at the minimum allowable evaporating temperature is
approximately 10 tons. If the system had to be on the line
down to zero load, the bypass valve would have to bypass 10
tons of hot gas. With the necessary system factors — R-22,
26°F evaporating temperature at the reduced load condition,
and 80°F condensing temperature — the capacity table is
checked for a valve which can handle the 10 ton requirement.
The SDR-4 has sufficient capacity for this condition. Both
7/8” ODF and 1-1/8” ODF connections are available on the
SDR-4 for piping convenience, and the proper size should be
specified when ordering. See ordering information.