Traditionally, compressors for commercial refrigeration have been sized with cooling capacities sufficient for the greatest system load they will encounter (i.e., pull down in a refrigerator application). But because loads vary widely with the seasons, and even from hour to hour, the compressor sized for the maximum load turns out to be oversized for long periods.
On average, commercial systems run partially loaded about 65 percent of the time, delivering full capacity for short periods and then shutting down.
When compressors are sized for more capacity than is needed to meet load conditions, the cost in wasted energy is significant. Also, for some time just prior to each cutout, temperature overshoot occurs on pull down - the compressor provides more cooling than needed by the product or process it serves. More frequent cycling results in additional wear, which can reduce the compressor's service life.
In larger commercial systems, various strategies have been used to more closely match capacity to the changing system load at any given time. Multiple compressors can be staged on and off to provide capacity steps. For example, a rack using two 20- and two 10-hp compressors can supply from 10 to 60 hp. This is done in 10-hp stages by operating various combinations of the available compressors. Compressor unloaders have also been used to reduce the capacity delivered by a compressor.
The advent of variable-frequency drives for refrigeration compressors has made an infinite number of capacity steps possible. With proper control, capacity can be continuously varied to exactly match system load at all times. The increase in energy savings with continuously variable capacity is dramatic, temperature overshoot on pull down is eliminated, and decreased cycling lowers maintenance costs significantly.
Until recently, though, variable-capacity technology has not been available in a wide range of commercial applications. Such uses include walk-in coolers, compressed air driers, heat pumps, and virtually any application requiring a condensing unit.
HOW VARIABLE CAPACITY FUNCTIONSMotor rotation speed is proportional to line voltage frequency. In order to change speed and compressor capacity, it is only necessary to change the frequency.
The VCC (variable-capacity compressor) has an integral electronic controller that takes temperature and pressure input from system sensors and automatically adjusts the compressor capacity to match the load. It uses algorithms designed, in the situation described here, by Danfoss for refrigeration and air conditioning applications.
The manufacturer offers variable-capacity compressors that can deliver 50 percent to 150 percent of nominal capacity, and can run constantly at any capacity within those limits. The units have been designed to handle lubrication at all speeds.
Danfoss has engineered type MTZ compressors specifically for operation with its AKD Adap-KoolÂ® drives, and delivers variable-capacity compressor solutions that include both products as a matched package.
For low- and medium-temperature applications, the company has nine packages including drive and matched compressor (nominal capacities from 3,900 to 90,500 Btuh (as rated by ASHRAE MBP). The new variable-capacity compressors are called the VTZ family.
With no predefined capacity steps built in, the VTZ compressor progresses smoothly from minimum to maximum capacity as needed. The desired evaporating temperature and room or process temperature is closely maintained with minimal fluctuations.
Reduced cycling decreases mechanical and electrical stresses on the compressor, and the AKD drive provides an inherent soft start that further reduces stresses.
ENERGY EFFICIENCYDuring 65 percent of its service, when a compressor is running under partial load, conventional fixed-capacity compressors generally display lower efficiency than at full load. In the case of a variable-capacity compressor with a matching drive, the compressor operates at high efficiency regardless of the load. Consistent high efficiency is made possible:
Compared with a single-speed model, the variable-capacity compressor can be several sizes smaller to match the nominal system load. The compressor operates at full efficiency throughout its capacity range. Capacity of the smaller compressor is easily reduced, resulting in high efficiency at low system load.
Whenever the compressor is operating at partial load, the condenser is oversized, and condensing pressure is lower. Consequently, required power input to the compressor is lower.
Promising applications include air driers, walk-in boxes and cold rooms, heat pumps, and virtually any low- or medium-temperature condensing unit application. Many different refrigeration and process cooling applications have thermal load profiles that suggest their suitability for innovative variable-capacity compressors with variable-frequency drives.
Sidebar: Storage, Process CoolingIn a multi-evaporator cold room with significant seasonal and daily load variations, and with an average system load of 68 percent, a Danfoss VTZ package produced energy savings of 26.2 percent, the company said. The VTZ compressor achieved a COP that was 140 percent that of the fixed-capacity compressor. Evaporating temperature was 23°F with R-404A. Maximum design cooling capacity of the fixed-capacity system was 75,000 Btuh.
In another installation, an industrial process cooling application running seven days a week produced energy savings of more than 60 percent and a COP 250 percent that of a fixed-capacity compressor. This system required continuous compressor operation in order to meet rapid and frequent demand changes, with very large load fluctuations throughout each day.
Hot gas bypass unloading reduced load to 20 percent of nominal. Average system load was 50 percent. Evaporating temperature was 37°. Maximum design cooling capacity of the fixed-capacity system was 102,500 Btuh.
Publication date: 04/03/2006