Variable Speed Control

Equipment is sized for peak capacity requirements, but often it is oversized for the existing load. In this situation, fans, pumps, compressors, and other rotating equipment waste electrical energy. Dramatic savings can result from reducing the motor speed to match the capacity of the equipment to the load. This is often accomplished by the installation of a variable frequency drive, which is an electrical device that is designed to operate standard AC induction motors over a speed range of 0-100%. Some examples where this technology has been applied follow:

In refrigeration applications, evaporator fans are often continuously operated at full speed even when the heat load is minimal. In this situation the fans produce approximately 70% of the refrigeration (heat) load. In addition to the negative impact that full capacity fan operation has on energy efficiency, the product quality can also be adversely affected. Research has shown that excess evaporator capacity tends to result in less stable temperature control, which is detrimental to highly perishable products. Also, the high air velocity that results from full speed fan operation can seriously affect the rate of moisture loss, or transpiration, from the product. This equates to less product left to sell (as much as 2-5% weight loss) and lower quality due to shrivel.

Variable speed evaporator fan control is a technique that typically operates the fans over a range of 40 - 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. In addition, the reduction in energy required to drive the fan motors results in less heat being added to the refrigerated space, thus reducing the load on compressors and condensers as well. Refer to the graph to the right for a better understanding of this fan power curve.

Variable speed technology is very well suited for refrigeration compressor and condenser fan applications as well. The system uses intelligent control algorithms to operate these critical components at the speed required to match the refrigeration load, eliminating the need to continually start/stop the motors to control capacity. Also, the compressor slide valve can remain 100% loaded while the motor speed varies for maximum efficiency.

Some pump applications can also benefit from variable speed control. In low static-head pressure installations, the pump speed can be varied with the flow requirement to achieve energy savings similar to fan applications.

The advantages offered by variable speed control are as follows:

• maximum energy savings
  
• full time operation of equipment resulting in uniform operating conditions
  
• reduced moisture loss from highly perishable product
  
• reduced equipment wear
  
• greater personnel comfort
  
• reduced equipment noise
  

If your plant isn't benefiting from variable speed control, you could probably earn some “hero buttons” by realizing significant energy savings through well planned implementation of variable frequency drive technology.

Duty Cycling

Duty cycling is a low cost method of reducing capacity of equipment. Some industrial applications permit on/off cycling of equipment to achieve energy savings and match the capacity to the load. In the case of refrigerated storage facilities, the control system provides the capability of cycling zones off at regular intervals. An override feature will restart refrigeration if the zone temperature rises significantly. Adjustable refrigeration on/off schedules are easily entered from a graphic screen. A typical schedule would only operate the evaporators a small fraction of the time.

Does your facility have equipment that can be duty cycled? How much could your efficiency be increased using this simple technique?

Demand Refrigeration

Demand refrigeration is a method of saving large amounts of energy on facilities that permit turning off the refrigeration system for several hours at a time. This technique is different from duty cycling where refrigeration is shut off in individual zones on scheduled intervals to reduce energy consumption. With demand refrigeration, the entire facility's refrigeration system is shut down when all the zone temperatures are reasonably close to setpoint. The system remains off until any single zone requires refrigeration again, indicated by a rise in temperature. At that point, refrigeration is resumed in the entire system until the zones are all back to the desired temperatures. This cycle is continuously repeated.

Intelligent Engine Room Control

You won't believe how simple it is to select capacity control and compressor sequence parameters to fit your plants changing control needs. You decide which compressors will run under different load requirements - even define parameters for auto-suction pressure reset control. Make sure that you have the capacity you need, when you need it, without wasting energy and without accumulating unnecessary run hours on compressors.

If you don't use wet-bulb approach to determine optimum discharge pressure, you are undoubtedly working your compressors harder than necessary. Efficient condenser control is as simple as tracking ambient wet-bulb temperature (the ideal limit of evaporative cooling) and continuously resetting the discharge pressure setpoint to utilize the currently available condensing capacity. Just add variable speed control of condenser fans and really achieve optimum energy efficiency.

Load Shedding

Load shedding is another energy feature offered by Techni-Systems. Selected loads can be cycled off at a preset time of day, and back on at a later time to reduce energy consumption and time-of-day demand charges. One set of times may be used for weekdays and another for weekends. This same scheme can be used to lower fan speeds or whatever makes sense for your plant operation.

Demand Limiting

In order to reduce demand charges, the rate of kilowatt usage can be monitored by the system, and control algorithms implemented to keep the demand from exceeding a maximum limit. As the electrical demand approaches the limit, certain equipment can be inhibited from starting or further loading. If the demand rises above the limit, equipment will begin to be unloaded or cycled off. When the facility load returns to a normal level, the equipment will return to normal operation.

Intelligent Defrost Control

Refrigeration systems typically defrost evaporator coils at preset intervals. This often results in unnecessary defrosting, which actually adds heat to the refrigerated space and reduces the time available for product cooling. In some cases, defrost cycles have the effect of reducing the moisture content of highly perishable products. Automated on-demand defrost control can not only save energy but also improve product quality and weight. On-demand defrost control is designed to hold off defrost cycles until they are necessary as indicated by the performance of the evaporator coil. A demand defrost condition is detected when the zone temperature is above setpoint and the temperature drop of the air passing through the coil is less than normal. If these conditions exists for a preset delay period, a defrost cycle is initiated.

Improving energy efficiency is our bag, and we've got lots of tricks! Techniques such as duty cycling, demand refrigeration, intelligent engine room control, load shedding, and, of course, variable speed control will greatly reduce your energy costs. Many facilities have been using these techniques for years. How about your plant?