Refrigeration Systems and Variable Speed drives

As with most systems, refrigeration systems are designed for worst-case full-load conditions (At high ambient temperature, high humidity). However, most of the time, the loads are average, not peak, and full capacity is not required. During average conditions, condenser motors in traditionally designed systems (without variable speed drives -VFD) either constantly are running at a higher speed than necessary or frequently cycling on and off. Producing more capacity than needed wastes considerable energy, and frequent on/off cycling accelerates wear and shortens the useful life of motors, contactors, and other components. Frequently starting and stopping motors and continually accelerating them to full speed eliminates opportunities for reducing energy costs. VFDs can help in both of these areas as well as provide better product environments.

In many applications, full load conditions exist for as little as 10 percent of running time . It is desirable for systems to provide as much of the required capacity for expected part-load conditions as possible. Considerable energy is wasted to meet peak demands when part-load conditions exist, especially if machines simply are switched on and off.

Benefits

Variable Speed Drives should be applied to regulate the operation of remote condenser fans to provide constant load-matching capacity and saving energy by eliminating over-capacity running. In addition variable-speed operation can result in substantial maintenance savings and enhanced compressor and fan-motor lifetimes. Variable speed also solves the noise-abatement problem for condenser fans. It is important in retrofitting variable-speed installations to use inverter-rated motors.

Condenser Fan Speed Control

Condenser-fan control can be applied to air-cooled or evaporative condensers. Drive controls compare condensing temperature with outside air or wet-bulb temperature so only the capacity needed to reject the heat used to meet the condenser design temperature differential is required.CondenserFan

As mentioned previously, VFDs applied to condenser fans provide not only the benefits of energy savings and capacity control, but the benefit of reducing noise. Before VFDs were available, local authorities often made building costly acoustical barriers to shield a store's surroundings from the noise of its condenser fans at full speed, necessary. Sometimes, owners were forced to relocate condenser units at great expense.

A VFD on a condenser, fans now can run at a lower speed, reducing startup noise. Also, the whine that is produced as fans start and climb rapidly to full speed also is reduced because the fans are ramped up to the required speed gradually.

Head Pressure Control

Head pressure control is required for the proper operation of most all direct expansion (DX) refrigeration systems that will operate at significantly lower ambient conditions than the full load design. The purpose of head pressure control is to artificially keep the compressor discharge pressure at a high enough operating condition to maintain proper stable system operation.

When the outside air temperature drops, the condenser head pressure drops. With a lower condenser head pressure, the condenser capacity increases. Energy consumption is reduced.

In Northern climates, such as Canada, variable speed control of single phase motors is common in supermarket and data center applications. In these applications condenser fans are controlling head pressure via a pressure transducer.

Johnson Controls offers a wide range of head pressure controls for all types of refrigerants -from on-off control to variable speed. Contact Yorkland Controls for your refrigerant control needs. (info@yorkland.net)

VFD Application Guide

Johnson Controls P266