Low Ambient Head Pressure Control

Low Ambient Head Pressure Control

within the operating limitations of the system, it ir desirable to take advantage os lower condesing temperatures whenever possible for increased capacity, lower discharge temperatures, and lower power requirements. However, too low a discharge pressure can produce serious malfuntions. Since the capacity af capillary tubes and expansion valves is proportional to the differential pressure across the capillary tube or valve, a reduction in discharge pressure will reduce its capacity and produce a drop in evaporating pressure.

Low discharge pressure can result in starving the evaporator coil with resulting oil logging, short cycling on low pressure controls, reduction of system capacity, or erratic expansion valve operation.

Systems with water cooled condenserd and cooling towers require water regulating valves, or some other means of controlling the temperature or the quantity of water passing through the condenser.

If air cooled air conditioning systems are required to operate in ambient temperatures below 60º F., a suitable means of controlling head pressure must be provided. Refrigeration systems are also vulnerable to damage from low head pressure conditions, and adequate head pressure controls should be provided for operation in ambient temperatures below 50º F.

Several proprietary control systems are available for low ambient operation, most of which maintain head pressure above a preset minimum by partially flooding the condenser and thus reducing the effective surface area. Methods of this type can control pressure effectively, but do require a considerable increase in refrigerant charge and adequate receiver capacity must be provided.

Air volume dampers on the condenser operated from refrigerant discharge pressure provide a simple, economical, and effective means of control which is widely used.

Adequate protection a lowest cost can often be provided by a reverse acting high presure control which senses discharge pressure, and acts to disconnec the condenser fan circuit when the head pressure falls below the control´s minimum setting. The proper adjustment of the off-on differential is particularly important to avoid excessive fan motor cycling, and the resulting fluctuations in discharge pressure may contribute to uneven expansion valve feeding. In cold ambient temperatures the condenser must be shielded from the wind.