Balancing the refrigeration/energy equation
01/01/2009
Unavoidable factors can turn troubleshooting into a crap-shoot for the service technician. Often, however, the problems encountered originate with the design process. Its not a lack of skill on the designers part, but the complexity of factors that affect each and every refrigeration installation make each job a new experience. No one is capable of anticipating the entire complex climatic, zonal, structural, product and human factors that change prior to, during and after construction, especially when the process is an as-built. Years ago I had the unfortunate task of following a fellow journeyman to a job where he was doing routine service-contract work. Unfortunately for him and our employer, the job was a disaster. I arrived the day after the service work to a complaint of a noisy air conditioning unit. Briefly, what I found was the two-inch (about 50 mm) fan shaft worn down to about ? inch. (See figure 1.) The bearing, its housing and a good part of the fan housing were missing, ground into a fine metal dust. The filter bank was so badly plugged with dirt that the filter sections could not be removed through the insertion slot. It was necessary to dismantle the whole fan unit. Consider the cost: 1. energy wasted, both electricity and gas, for what had to be years of neglect as no one working this service call could have checked, let alone cleaned, the unit. 2. The cost of the equipment and the energy it takes to replace the damaged parts. There was not even a savings on buying filters because they were washable. The fan housing was salvaged only because the company had a sheet metal division that could fashion some repairs. Needless to say, the company lost the contract and the employee lost his job. More importantly, in the grand scheme of things if service technicians do not effectively provide service huge amounts of dollars in energy losses can occur each year.
Design and service planning
One must always keep in mind the refrigeration processes viewed through a pressure enthalpy diagram (PE Diagram). If the diagram is not used, then the calculations associated with the diagram should be processes as a minimum. The PE diagram provides us with a pictorial view of the system as it will operate. Systems live and expire based on their coefficient of performance (COP). COP is the energy extracted by the evaporator, net refrigeration effect (NRE), divided by the energy required to do the work, heat of compression (HoC). COP is related to SEER. Heat of Rejection (HoR): this is the heat rejected by the condenser. If the condenser is too small the system suffers, since the HoR must remove the heat of NRE and HoC. Too large and the cost to purchase and operate adversely affects energy consumption. Subcooling: Subcooling is generated by reducing the amount of sensible heat in a liquid below its saturation point. The more heat removed from the refrigerant prior to entering the metering device, the more heat can be picked up by the evaporator. Superheat: Heat added to a refrigerant vapour above its saturation point is superheat. It has two major functions and can create some problems. The positive functions are: a. Protecting the compressor from liquid refrigerant slugging. b. Providing a control base for thermostatic expansion valves. On the negative side: a. Too much superheat in the evaporator robs the system of refrigerating capacity. b. Too much suction line superheat increases temperature penalty (TP). (TP equates to a pressure loss in the suction line and should not exceed 2?F. Moreover, an increase in suction line superheat adds increased stress on the compressor with high temperatures and increased vapour volume. Volume of Vapour Pumped: the volume of vapour pumped is critical. Volume increases with increased suction temperature, either from superheated refrigeration or a lack of suction line insulation, and it increases as the suction pressure drops. An increase in the volume of vapour pumped means a decrease in the mass of refrigerant pumped and that equates to wasted energy. Compression Ratio (CR): Compression ratio the absolute discharge pressure divided by the absolute suction pressure is critical; the higher the value, the lower the efficiency. It increases if suction pressure drops or discharge pressure increases. Often these pressure changes are a direct result of dirty coils.
Energy consumption awareness
Another concern for energy consumption with respect to design includes technicians spending 80 percent of their time on 20 percent of the causes of energy loss. Consider that energy use in a large air conditioning system is: 1. 35 to 40 percent by refrigeration; 2. 40 to 55 percent by fans; and 3. 20 percent by pumps and towers. As mentioned earlier, fans tend to be neglected, but they are among the highest energy users in a building. So when designing, plan for energy consumption with the following in mind: 1. Size equipment to supply air at a lower temperature to reduce fan speed and energy costs. 2. Match air handler, fan motors and duct sizes to the lower velocity air. This could aid in humidity control as well, especially for grocery stores. 3. The humidity in the building would be lower, allowing an increase in set points. 4. Increasing the initial equipment costs will provide long-term payback. 5. This could increase the size of the refrigerating equipment and reduce the amount of economizer time. 6. DX coils and drains would increase in size. 7. Compressor size would increase and some form of capacity control or hot gas bypass should be employed. 8. Consider heat recovery, heat pumps and geothermal processes where feasible. 9. Consider using scroll or screw compressors instead of the traditional reciprocating compressors. 10. Minimize condenser temperatures for energy-efficiency gain and compressor longevity, especially reciprocating compressors.
On the refrigeration side energy use differs, with 1. 75 to 85 percent by refrigeration; 2. 10 to 20 percent by fans; and 3. five percent by pumps and towers. Obviously here, attention to compressors is critical. Also it is important to apply service to those major areas that affect energy consumption. (See Figure 2)
Finally, when installing or servicing refrigeration systems, consider the effects of refrigerant leaks and over or under refrigerant charging. Figure 3 compares efficiency with the percent of full refrigerant charge. Whether under or over, the result is a loss of energy. Further to that loss is the callback and risks to the environment. A diligent approach is a must.
John Carr is a mechanical trades instructor at SAIT Polytechnic in Calgary. He can be reached at john.carr@sait.ca.
