Chilled water plants range from the simple one chiller and one pump system to multiple chillers and multiple pumps. In many facilities the chiller plant is the major energy consumer and sets the energy rates charged by the local utility year-round. If you are a facilities operator or plant manager your job is to assure that the plant operates properly and provides comfort to the building occupants. As long as there are no complaints from the occupants, the plant is assumed to be operating properly. So, where does chilled water plant efficiency fit into this?
Components of the Plant
The chilled water plant consists of a chiller, a chilled water pump, a downstream heat load, controls and the interconnecting piping. Smaller plants might use an air-cooled chiller which requires no cooling tower or condenser pumps, but the larger systems always use them for the added efficiency. Generally, plants over 300 tons generally use cooling towers and condenser pumps. Put all this equipment in one room, pipe it up and connect it to the electricity sources and you have a chilled water plant. Sounds easy and straightforward when put into these terms, but the plant has a life of its own and the cost of this life is often paid to your local electrical utility with a few dollars thrown in for the maintenance staff.
The average person would think that the plant with the most efficient equipment would be paying the smallest electrical bill. After all, the most efficient KW/ton chiller and the extra cost of purchasing that higher efficiency has to be justified in the end with a shorter payback period. Sadly, this is not the case. Most plants don’t even come close to operating at maximum efficiency. Once the plant is up and running, generally there is no one to critique the actual plant operating efficiency. Two chillers operating at part load, chilled water temperatures not approaching designed difference and variable speed pumps that never ramp down below 85% are all symptoms of chiller plant inefficiencies. So, what does the plant operator do to increase his chiller plant efficiency?
Measuring Chiller Plant Efficiency
The first and most obvious measuring stick is the chiller plants current efficiency at various operating points. Chiller plants are designed to satisfy the maximum cooling load of the facility but rarely operate in this range for long periods of time. Temperature swings from summer to winter and from day to night cause the plant to operate at part loads most of the time. Measuring the chiller plants efficiency is somewhat simple and easy to accomplish assuming that most plants use Direct Digital Controls. The task is to determine the KW per ton of cooling. The KW is the sum of the power inputs to the operating equipment and the tons of cooling is the output of the chiller plant. This efficiency will vary through the course of time in the same sense that the outside air temperature varies from hour to hour.
Determining the power input of the plant equipment is fairly simple and consists of gathering data for the chillers, condenser pumps, tower fans and chilled water pumps. Most of the time the nameplate data can be used for pumps and fans unless they have variable speed (frequency) drives. Chillers, on the other hand, vary their power consumption dependant upon the loading, condenser water temperatures and chilled water temperatures. Summing all the power input to the components determines the instantaneous total power input for the plant. Note that the control system offers big advantages for power input measurements depending upon the sophistication and number of input/output points. Power meters on each piece of equipment or outputs from the variable speed drives which input the instantaneous power usage to the control system makes this measurement easier and more reliable.
Determining the output of the chiller plant is a basic process also with the right equipment. Plant cooling capacity varies as the chilled water flow rate and the leaving and returning water temperatures. Changing either of these affects the total plant capacity. The equipment required for this measurement consists of a water flow meter and two water temperature sensors. One sensor to measure chilled water supply and another sensor to measure chilled water return temperature. The sensors may already be installed in the piping system and reporting back to the control system. The quality of the water flow meter is very important and due to the cost of installation, should be of the highest quality. If the plant can’t shut down or the piping section can’t be isolated, a clamp-on ultrasonic meter is probably the best alternative. The plants total cooling capacity in tons (12,000 BTUH/HR=1 Ton of cooling) is:
Tons = (chilled water return temperature minus chilled water supply temperature) times chilled water flow divided by 24 or;
Tons = (ΔT X GPM) / 24
Temperatures are in degrees Fahrenheit and water flow is in GPM.
The chiller plant efficiency is then determined in KW / Ton.
Comparison of Plant Efficiency
Actual plant efficiency measurements tend to fall above 1.0 KW/Ton ranging up to 1.4 KW/ton. A very efficient plant will range at 0.7 KW/ton to 0.8 KW/Ton with mid-range efficiencies at 0.8 KW/Ton to 1.0 KW/Ton. If the plant has higher efficiency chillers and moderately sophisticated digital controls, the plant efficiency can be improved for any system operating at 0.9 KW/Ton and above. If the chilled water plant is older and equipment is near the end of its useful life, the increased efficiency potential is even larger.
Increasing Plant Efficiency
Each chilled water plant is different and the methods used to increase efficiency can only be determined after an investigation of site conditions and operating methods. Site conditions include pumping methods, location of chillers, chiller efficiencies, downstream control valve characteristics, water temperature measurements, etc. Operating methods include chiller staging, cooling tower staging, chilled water set points, experience of maintenance staff, etc.
Two chillers operating at part load conditions, chilled water temperatures not approaching designed difference (normally between 10 and 12 degrees) and variable speed pumps that never ramp down below 85% are all symptoms of chiller plant inefficiencies. Review your power bill for the chilled water plant equipment and imagine lowering it 20-30%. Review the rate schedule your local utility charges for power and where your current demand falls and consider a third-party review of your chilled water system. We can help.