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Low Ambient Operation
Refrigeration circuit components are sized for the most
demanding application point. This is typically when the
ambient temperature is high and the evaporator temperature
is low. Many systems are required to operate properly when
the ambient temperature is much lower.The issue here is
the condenser becomes "too efficient" and lowers the liquid
temperature and pressure beyond the range that the TX valve
compensates for. In these applications, some form of low
ambient control is required.
Water-cooled systems typically use some form of condenser
water bypass line to maintain head pressure. For air-cooled
systems, there are three common approaches to design for low
ambient operation:
• Fan cycling
• Fan speed control
• Condenser flood back
AG 31-011 • REFRIGERANT PIPING DESIGN
Fan Cycling and Fan Speed Control
Fan cycling and fan speed control are the most common
forms of low ambient operation for commercial air conditioning
systems. Fan cycling entails staging condenser fans ON and
OFF based on the ambient temperature or the head pressure.
Ambient-based control is cost-effective, but should only be
used with air conditioning applications. Process loads must use
pressure-based controls.
Fan speed control entails using some form of fan speed
controller to modulate air-flow through the condenser. Fan
speed is usually based on head pressure.
Both of these approaches are options provided by the
equipment manufacturer and have minimal impact upon the
piping system design, other than requiring a port for the head
pressure sensor.
Condenser Flood Back Design
Figure 20
shows a typical condenser flood back arrangement.
As the ambient temperature drops, the head pressure drops.
A head pressure control valve (refrigerant flow regulator)
controlled by head pressure begins to close, restricting flow of
liquid refrigerant from the condenser. Liquid refrigerant "floods"
the condenser. As the tubes in the condenser flood, they
reduce the surface area available for condensation and reduce
the heat rejection capacity. As the condenser floods, the
head pressure climbs until it reaches the setting of the head
pressure control valve.
Single head pressure control valve arrangements are available
from several manufacturers. Refer to the manufacturers
instructions for sizing and applications.
A receiver is required to store refrigerant during warmer
weather. The receiver should be sized so it is at 80% of
capacity while containing the entire system charge. Another
aspect of receivers is that they contain both liquid and gaseous
refrigerant at the same time. By their design, receivers prohibit
liquid subcooling from occurring. Without liquid subcooling the
capacity of the system is reduced and care must be taken in
the design of the liquid line to avoid flashing at the TX valve.
Flooded systems are an excellent method of providing head
pressure control in cold climates, but they increase initial
cost, add complexity to the refrigeration system, increase
installation time, and increase the refrigerant charge. The loss
of subcooling should be recognized. If a flooded system is
required, consult the manufacturer prior to installation.
Tip: Flooded systems with receivers are
complex. Consult the manufacturer for assistance
before installing.
34
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