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Article - January 23, 2001
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Thomas Hartman, P.E. Contributing Editor

This follow up article to the January 1st article "Improve Chiller Plant Efficiency" describes the steps I recommend to improving chiller plant efficiency in response to several readers who asked me that question. 

Control Solutions, Inc Earlier this month I wrote about an opportunity to improve the efficiency and lower the operating costs of chiller plants with a better focus on plant efficiency. Traditional chiller plant operations rarely consider efficiency. This commentary struck a cord with several readers who contacted me to ask how they should go about improving their plant's efficiency. Here, I will outline my recommended step by step approach for moving ahead to a more efficient plant.

The old saying "If you can't measure it, you can't control it!" applies to chiller plants. Generally, chiller plant efficiency is based on the nominal efficiency of it chillers. This is not an effective means for determining plant operating efficiency because chiller plant efficiency has more to do with how the equipment is controlled than the nominal efficiencies of the individual components. Here are the steps I recommend to move toward more efficient chiller plant operations.

STEP 1: MEASURE YOUR PLANT'S OPERATING EFFICIENCY

The first step in improving chiller plant operating efficiency is to determine how efficiently the plant is now operating. I recommend installing equipment to provide real-time plant efficiency measurements so that the operations staff can at view the current plant efficiency in kW/ton or C.O.P. (coefficient of performance). At last month's ASHRAE Winter Meeting, I presented a paper on this issue. If after reading this article you want more information on how to measure and compute plant efficiency, I suggest you obtain a copy of that paper through ASHRAE.

Measuring plant performance requires measuring the chiller plant output in tons of refrigeration (1 ton = 12,000 BTU/hr) and measuring the total input in kW. Dividing kW input by the tons output provides the kW/ton of an operating plant. If it is preferred to measure efficiency in C.O.P., simply divide 3.51 by the kW/ton value, and the result is the C.O.P. of the plant.

Power input for a chiller plant is usually easy to obtain. The chilled water generating equipment for a chiller plant includes the chillers, condensing pumps and tower fans, but not the chilled water pumps. Since many condenser pumps and tower fans are constant speed, all one need do is make a single power (kW) measurement of the pump or fan in operation and apply that power anytime the unit operates. The status of the device (e.g. "ON" or "OFF") then is employed to determine the kW for each such pump or fan. Chillers vary in power requirement as the loading changes, and the power factor of the chiller changes, too. There are two approaches that can be employed to find the kW of a chiller. First, a power meter can be installed on each chiller (or for all chillers if the chillers receive power from a single dedicated transformer or motor control center). A less costly approach is to measure the power factor at various chiller loading points, to use the current transducer supplied with the chiller and to apply the power factor as a function of current draw along with line voltage to calculate the power draw over the load range of the chiller. kW input for the plant at any time is simply the sum of the kW draw of the on-line chillers, condenser pumps and tower fans.

Measuring the output of the plant requires the installation of a flow meter on the chiller water supply line and two temperature sensors on the chilled water supply and return lines. Chiller output in tons is calculated as:

Tons = ( chilled water return Temp (oF) - chilled water Supply Temp (oF) ) x Flow (gpm) / 24.

For the flow meter, I recommend either a high quality turbine type or a magmeter. The magmeter is more expensive but requires no maintenance and is more accurate, especially if your plant does not have a long straight length of piping in which to install the flow meter. It is very important that the supply and return chilled water temperature sensors be precisely calibrated. I recommend installing them close enough together and/or with leads long enough to permit periodic calibration in the same ice water bath. Remember, for calculating plant efficiency, the actual temperature of the chilled water is not as important as the difference between the supply and return temperatures. By calibrating the two sensors in a single water bath so that they read exactly the same temperatures, you can be certain the plant output calculation will be accurate.

The plant or building digital controls can be employed to read the values and make the calculations necessary to determine the plant efficiency in terms of kW/ton or C.O.P. By self-installing the equipment and assisting or completing connection to the building control system, you should be able to get the required instrumentation installed and operating to provide plant efficiency for several thousand dollars or less. I recommend that a display screen be installed in the chiller plant so that operators have direct access to plant operating efficiency at all times.

STEP 2: USE A BENCHMARK TO EVALUATE YOUR PLANT

With a real-time efficiency reading for your plant, one of your first activities should be to see how your plant compares with a benchmark so you can plot the right course toward improving its efficiency. The figure below is a rating system we use for centrifugal chiller plants. Since centrifugal chillers are more efficient than other types of chillers, you may need to adjust this scale upward about 0.05 to 0.10 kW/ton for screw or scroll compressors and about double that for reciprocating compressors. You may also wish to make some adjustment for your local climate. If you are located in the Southeast US, your annual average kW/ton will be a little higher and if you are located in a temperate climate such as the Pacific Northwest, it will be a little lower.

Average annual chiler plant efficiency in kw/ton (COP) Chart

As shown in the above Figure, any chiller plant that operates continuously above about 1.0 kW/ton (before chiller and climate adjustments) is in need of improvement. A plant operating in the red range of the Figure can almost always incorporate improvements cost effectively. The place to start is in trying new operations strategies.

STEP 3 USE YOUR MEASUREMENT TO IMPROVE YOUR OPERATIONS

If the comparison of your plant's efficiency with the above benchmark shows it to be operating well into the red zone, you may wish to start immediately to consider certain redesign or upgrade options. However, if it is operating only a few tenths of a kW/ton higher than the benchmark, I recommend you work on improving operations to bring that value down. With efficiency monitoring, it is a simple task to improve your plant operations. Consider determining when a chiller should be added or shed. If the efficiency is improved when you add a chiller, you should add the chiller sooner. If efficiency falls, you have added the chiller too soon. You can similarly work with tower and tower fan operation, and you can see what effect adjusting chilled water supply temperature has during lower load periods. However, keep in mind that adjusting the chilled water temperature may affect the pumping power requirements that are not included in chilled water plant efficiency monitoring.

STEP 4 CONSIDER CONTROLS OR EQUIPMENT CHANGES

Once you have some experience with improving operations, it may be time to explore more fundamental changes if you plant is still operating in the red zone much of the time. My experience is that a plant operating in the red zone can almost always be improved cost effectively. Outside help is probably the best way to proceed at this point as your organization will benefit from the fresh thinking it provides. And remember, control improvements are almost always the most cost effective improvements. Unless the equipment is reaching the end of its operating life, it is almost never wise or necessary to replace equipment to get the plant efficiency under control!

Additional information on technologies discussed in this article is available at www.hartmanco.com . Comments and questions about the article may be addressed to Mr. Hartman at tomh@hartmanco.com


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