Bally Refrigerated Boxes BVC Generation C – Direct Drive Air-Cooled – Vertical Air - Manual (Page 30)

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LOW AMBIENT OPERATION
GENERAL
When a remote air cooled condenser is installed
outdoors, it will be subjected to varying tempera-
tures. Within many areas, winter to summer annual
temperatures swings can be as high as 120
o F or
so, this will have a major impact on the perform-
ance of the condenser. As the ambient temperature
drops, the condenser capacity increases due to the
wider temperature difference between ambient and
condensing. As this happens, the condensing
temperature also drops as the system finds a new
balance point. Although the overall system capacity
will be higher at lower condensing temperatures,
other problems can occur. The capacity of an
expansion valve is affected by both the liquid
temperature entering the valve and the pressure
drop across it. As the condensing temperature
decreases, the pressure drop across the metering
device also decreases. A lower pressure drop
decreases the capacity of the valve. Although lower
liquid temperatures increase the capacity of the
metering device, the increase is not large enough to
offset the loss due to the lower pressure drop.
The following three sections cover the various
options used to control condensing temperatures.
(i) Fan Cycling
Cycling of the condenser fans helps control the
condensing temperature. With this approach to
solving low ambient problems, fans are taken
off-line either one at a time, or in pairs. It is not
recommended that multiple fan condensers cycle
more than two fans per step. The reason for this is
that the pressure in the condenser will increase
drastically as several fans are taken off-line at the
same time. This will result in erratic operation of the
refrigeration system and applies additional stress to
the condenser tubes. It is preferable to control the
condensing temperature as smoothly as possible.
Fans should be cycled independently on a
condenser where the fans are all in a single row.
On two row condensers, the fans should be cycled
in pairs.
Ambient temperature sensing controls can be set to
bring on certain fans when the outdoor temperature
reaches a predetermined setpoint. Pressure sens-
ing controls are set to bring on certain fans when
the condensing pressure reaches the setpoint on
the control. Temperature or pressure setpoints and
differentials should be set in such a way as to
prevent short cycling of the fans. Constant short
cycling will produce a volatile condensing pressure
while decreasing the life of the fan motors.
For recommended fan cycling switch settings, refer
to Table 4. Differential settings on fan cycling
temperature controls should be about 5
o F (2.8 o C).
On fan cycling pressure controls, a differential of
approximately 35 psig is recommended.
On supermarket applications (using 6-12 Fan
models) condenser fans may be cycled individually
(not in pairs) and therefore lower differential settings
may apply and will depend on the specific
application.
Fans closest to the inlet header should be permitted
to run whenever the compressor is running. If these
initial fans are wired through a cycling control, the
life of the condenser may be shortened due to the
additional stress placed on the tubes and headers.
Table 3 shows the fan cycling options available for
all condenser models.
(ii) Variable Motor Speed Control
If additional head pressure control is required beyond
the last step of fan cycling variable fan motor speed
may be used. Variable motor speed is optional on all
condenser models. A varying motor speed may be
accomplished using a modulating temperature or
modulating pressure control. A variable speed
controller can be an electronic or solid state device
which varies the voltage going to the motor
depending on the temperature or pressure of the
medium being sensed.
(iii) Refrigerant Regulating Controls
Pressure regulating controls are available from a
number of valve manufacturers. The purpose of
such a control is to regulate the refrigerant flow in
such a way as to maintain a pre-selected
condensing pressure. In lower ambient tempera-
tures, these valves throttle to maintain the desired
pressure and in doing so, flood the condenser with
liquid refrigerant.
The larger the condenser surface is, the higher its
capacity will be. When a condenser is flooded, its
useful condensing surface is reduced. This is
because the refrigerant occupies the space which
would otherwise be used for condensing.
Some control/check valve combinations will
regulate refrigerant flow depending on the pressure
at the inlet of the condenser.These are often referred
to as
inlet regulators. As the valve closes, hot gas
bypasses the condenser through a differential
check valve to increase the pressure at the receiver.