Auto Discharge Pressure Setpoint

Ambient temperature and relative humidity readings are used to calculate the wet bulb temperature, the ideal limit for evaporative cooling, which is then used to determine the discharge pressure setpoint. This setpoint is constantly recalculated to utilize all the available condensing capacity under the current ambient conditions. The minimum discharge pressure setpoint is user adjustable.

Condenser Fan Sequencing

Condenser fans sequence on when the discharge pressure is above setpoint for a time delay period, and sequence off when the pressure falls below setpoint. Condenser fans will typically be set up so the fans will come on in order of the smallest to largest. Under heavier load conditions this order can be reversed, allowing the larger fans to come on first. When two fans are of similar size, either one can be selected as the lead fan.

Variable Speed Control

Condenser fans can be operated over a range of 0 - 100% of full speed. This takes advantage of the cubic fan law, which is that the power required to drive the fan varies as the cube of the speed. In other words, the energy consumption of the fan motor drops off dramatically as the speed is reduced. Refer to the graph at the right for a better understanding of this fan power curve.

Variable speed technology is very well suited for condenser fans because all available fans can run together at the same speed, thus utilizing the entire surface area of the coils while dramatically reducing the power consumption. The system uses intelligent control algorithms to operate the fans at the speed required to match the refrigeration load, eliminating the need to continually start/stop the motors to control capacity.

The advantages offered by variable speed control are as follows:

• maximum energy savings
  
• full time operation of equipment resulting in uniform operating conditions
  
• reduced equipment wear
  
• reduced fan noise

Condenser Pumps

The user enters the start/stop pressures of each pump. Generally the pumps run all the time, but in cold weather the pumps will sequence off when the discharge pressure falls below the stop setpoints. When the pressure rises again, the pumps will sequence back on according to their start setpoints.

Non-Condensables

Non-condensables are detected in the system by comparing the discharge pressure with the pressure corresponding to the measured liquid condensate temperature. A gauge on the condenser control screen displays the non-condensable pressure to enable the user to know when purging is required.

A sample of one of our condenser control screens from a freezer storage facility in Woodland, Washington. This facility has two engine rooms, one of which has 4 suction headers.