Head Pressure Controls; Condenser Fan Cycling (Aa Models); Variable Condenser Fan Speed (Sa Models); Flooded Head Pressure Control - Trane TR-SCS Series Installation, Operation And Maintenance Manual

Introduction

Head Pressure Controls

Condenser Fan Cycling (AA Models)

Used for outdoor installations where ambient condenser
air inlet temperatures are 0 °F or higher, a condenser fan
cycling switch monitors refrigerant discharge pressure
and turns on the condenser fan as required to maintain
allowable condenser pressures. This is a high-pressure
differential control switch with Single Pole Single Throw
(SPST) contacts and an automatic reset. The switch
activates the condenser fan contactor to maintain
condensing temperature when the discharge pressure
rises. See Table 2, p. 8 for the fan cycling pressure control
settings.
Note: It may be necessary to alter the fan cycling control
settings on a case by case basis. This is due, in part,
to site specific heat loads and varying BTU
capacities of indoor evaporator (A/C) units. Contact
Trane Product Support for assistance.
On single circuit condensers, each fan is controlled with its
own fan cycling switch. Multiple fans are staged to operate
sequentially as discharge pressure rises. The primary fan
(closest to the header) turns on 1st as described above. If
pressure continues to rise, adjacent fan(s) are set to turn
on in sequential increments with the fan located furthest
from the header turning on last. Conversely, as discharge
pressure drops, the fans drop out sequentially in reverse
order.
Dual Circuit condensers employ a fan cycling pressure
control switch for each refrigeration circuit. For smaller
model condensers, two control switches (one per
refrigeration circuit) are adjusted to the same pressure
setpoint and are wired in parallel to operate a single fan.
Table 1. Fan cycling pressure control settings
Refrigerant Type
Cut-in
R407C 320 psig
R410A 440 psig
Table 2. Variable fan speed control settings
Refrigerant Type 1st Fan (Variable)
Cut-in
R407C 240 psig
R410A 340 psig

Flooded Head Pressure Control

Used for outdoor installations where ambient condenser
air inlet temperatures may fall to -30°F, flooded head
pressure control is used to maintain head pressure during
the low ambient temperature conditions. A head pressure
control valve and a receiver are used in the refrigeration
circuit to back up liquid refrigerant into the condenser coil.
The head pressure control valve is a 3-way modulating
valve controlled by the discharge pressure (see
8
1st Fan 2nd Fan
Cut-out Cut-in
240 psig 330 psig
330 psig 460 psig
2nd Fan
Cut-out Cut-in
315 psig 325 psig
440 psig 460 psig
Figure 3,
If either control switch senses a rise in pressure, the fan
will turn on. On dual circuit units with multiple fans,
operation of each additional fan requires two paralleled
pressure control switches (one for each refrigeration
circuit). Each set of paralleled switches will be set to the
same pressure such that the adjacent fans begin operating
at sequentially higher pressure increments. For larger
model micro-channel condensers (TR-SCS-MC-111
through TR-SCS-MC-334) the fan cycling pressure control
switches are not in parallel. The control switches operate
the fan(s) assigned to each refrigeration circuit
independently.
Variable Condenser Fan Speed (SA
Models)
Used for outdoor installations where ambient condenser
air inlet temperatures may fall to -20°F, a variable speed
condenser fan motor controller is used to maintain head
pressure. The fan speed control is a continual modulation
of the motor's speed. The condenser fan speed controller
monitors the refrigerant discharge pressure and as
discharge pressure rises, the fan speed increases. The
condenser fan speed varies as required to maintain
allowable condenser pressures. The fan speed controller
is set to maintain the correct condensing pressure. See
Table 2 for the variable fan speed pressure control
settings.
When used on systems with multiple condenser fans,
variable fan speed control is used only on the first fan
which is closest to the header. Additional fans use
pressure fan cycling control as described
Fan Cycling (AA Models)," p. 8
speed fan to maintain proper head pressure.
3rd Fan
Cut-out Cut-in Cut-out
250 psig 340 psig 260 psig
345 psig 475 psig 355 psig
3rd Fan
Cut-out Cut-in Cut-out
255 psig 340 psig 260 psig
355 psig 475 psig 365 psig
p. 9). The head pressure control valve and the receiver
may be located with the RCU or with the indoor evaporator
unit.
When the A/C unit begins to operate, the discharge
pressure rises. When the pressure reaches the "1st Fan"
pressure control setting ( See
is cycled on. If multiple fans are used, they will operate by
pressure fan cycling.
in"Condenser
, to assist the variable
4th Fan
Cut-in Cut-out
345 psig 265 psig
485 psig 375 psig
4th Fan
Cut-in Cut-out
345 psig 265 psig
485 psig 375 psig
Table 1), the condenser fan
SS-SVX006C-EN