Keith R. Cockerham, P.E., CUH2A
Chilled water systems for laboratories consist of three basic types: main (central) systems, process (laboratory equipment) systems, and specialty (environmental room) systems. The nature of this paper is to give the user a good overview of systems so that the reader can make informed decisions for their specific application. The objective is to be able to select the appropriate chilled water system for a particular laboratory application based upon efficiency, redundancy, and other project criteria. For a good fundamental overview of chilled water systems, one should start with the 2004 ASHRAE Handbook – HVAC Systems and Equipment, Chapter 38, Liquid-Chilling Systems.
There are four major manufacturers of electric centrifugal chillers in the United States. Manufacturers of rotary screw chillers include these four, plus one or two more. Scroll chillers are available from even more manufacturers, including a couple who specialize in laboratory applications.
The centrifugal chillers start from just below 200 tons, but do not become cost effective until approximately 500 tons. On the upper end, these units go to about 1400 tons on 480-volt power and can go up to about 2500 tons on dual compressor 480-volt machines. At higher voltages, single compressor units typically go up to about 3,000 tons. At least one manufacturer goes up to 5,800 tons on a dual compressor 4160 volt-machine. For higher tonnage applications, multiple machines are generally employed for better redundancy and to take advantage of better turn down capabilities and better part load performance. Variable frequency drives are available on the 480-volt machines and can offer considerable savings at part load performance.
For applications within large central chilled water plants, alternative prime mover chiller types are available. Steam turbine driven chillers are one type, and they range in size from 1,200 tons to 5,500 tons. Diesel engine driven chillers (natural gas or oil) are another type that range in size from 350 to 2,700 tons, as well as larger sizes from 2,700 to 5,500 tons. Additional alternative fuel chillers in smaller tonnage ranges include automotive/marine engine-driven chillers ranging in size from 50 to 1,000 tons and steam absorption chillers, single and double effect, that can go up to 1,000 tons.
These alternative fuel types are generally slightly less efficient and are considered less reliable, requiring more maintenance than the electric chiller types, but they offer the designer options for demand-side management. Electric tariff rates from electric utilities indicate that some chiller plants can be cost-effective if they incorporate chillers that use both electricity and an alternative fuel. Known as a hybrid plant, this mixing of chillers can offer significant cost savings when a central plant is operating in the high demand summer months.
A designer should also consider cogeneration—combined heat and power—for plant systems greater than 10,000 tons. Sequentially producing electricity and steam all year long can significantly reduce energy bills and realize significant savings in carbon dioxide (CO2) emitted into the atmosphere; up to a 70 percent reduction in CO2. One final demand-side management strategy to be considered for larger plants is the use of thermal (cold water or ice) storage. By producing cold water or ice at night when electric rates are cheaper, temperatures are cooler and equipment can run a little more efficient. The plant can then minimize or shut off chillers during the peak afternoons when electricity prices are highest.
Rotary screw chillers are typically sized from about 50 tons to 300 tons. They can go up to 500 tons, but because they are not as efficient as electric centrifugals above 300 tons, they are not commonly used above 300 tons. Part load performance and wintertime performance are particularly good with this type of chiller, especially when compared to a scroll or reciprocating air-cooled chiller application. Additionally, these chillers are highly reliable and their performance can be enhanced at part load by the addition of a variable speed drive.
Scroll compressor technology has almost completely supplanted reciprocating compressor technology. In smaller chilled water system applications, less than 100 tons, this type of compressor technology is very competitive with rotary screw technology. Scroll technology has evolved into an efficient robust technology. Air cooled chiller applications are common with this technology. Reciprocating compressors are still available but are fading because of efficiency and noise issues.
One of the newer technologies hitting the marketplace is magnetic bearing chillers. There are two manufacturers utilizing this technology, and chillers have been running commercially since early 2005. The claims of the manufacturers are being borne out in the marketplace: “Oil-free magnetic bearings provide quiet and reliable operation. Oil-free design reduces maintenance by up to 50 percent and eliminates the complexity, cost, and reliability issues of oil-based designs.” The part load performance of these chiller compressors offers a significant improvement in the marketplace. The compressors are available from 60 to 150 tons, and when installed in a multiple compressor arrangement, individual machines up to 900 tons are available. Water- and air-cooled machines are currently available.
Another technology that has been around a bit longer is evaporative cooled condensers. This technology works by spraying water over a finless condenser coil. The resulting evaporation assists in the rejection of condenser heat to ambient air. These units typically are pre-manufactured and shipped from the factory in tractor trailer truck sections and assembled at the jobsite. The advantages of this technology are that you get a smaller unit than an air cooled chiller with much higher efficiency; typically 35 percent less than an air cooled unit of the same tonnage rating. This means that it will require a smaller electrical feed, which in a renovation situation can mean eliminating the need for a completely new electrical service required by air cooled technologies. Clients also use this type of unit in a leased space; they take the chiller with them when their lease expires!
The following chart is taken from ASHRAE Standard 90.1-2004:
The above shows the MINIMUM efficiencies as required by the ASHRAE Standard which is recognized as code in most states. The standard is quite detailed in that it has multiple air water and chiller types indicated. Do understand the following terms:
- COP – Coefficient of Performance
EER – Energy Efficiency Ratio
IPLV – Integrated Part Load Value
ARI 550-92—This is the standard of the American Refrigerant Institute that derives the technical parameters that each chiller must be rated to (air and water temperatures).
The typical process loads that are encountered in a laboratory result from lasers, magnetic resonance imaging (MRI) machines, linear accelerators (oncology machines), computed axial tomography (CT) scan machines, and various types of bench-top equipment. These loads are generally taken care of by one of three systems: flow through, a closed loop heat exchanger, or a closed loop chiller. A flow through system is one where the water passes through the instrument (laser) once and does not return. Tap water is an example. A closed loop heat exchanger is a system where the cooling water circulates around a closed path that includes the instrument and a heat exchanger. The heat generated by the instrument is removed to the heat exchanger, which must itself be cooled by either a liquid or an air system. In a closed loop chiller system, the cooling water circulates around a closed path that includes the instrument and a chiller. The heat generated by the instrument is removed in the chiller, typically a refrigeration unit, which then discharges its excess heat into the air or into a liquid.
Process (Lab Equipment) Systems Process Chiller
Laser Lab Applications
The attributes required for these systems include precise temperature control, typically plus or minus 0.1° C. The water temperature operating range is typically from 5° C (41° F) to 35° C (95° F). Rugged construction and specialized particle water filtering is important. Typically, these systems operate above the dew point so as to minimize any condensation within the instrument being cooled, and they generally have an open reservoir design.
This diagram clearly shows some of the issues encountered with a laser laboratory. The heat exchangers (HX) are connected to a secondary chilled water system, which is generally well filtered to minimize any clogging of the instrument being cooled or of the HX itself. Most systems use “open” connections to the heat exchanger and require a solenoid valve and/or a backflow preventer to keep the system from draining out when the internal pump is shut down. Coherent is a common HX manufacturer and Neslab is a common chiller manufacturer of these systems.
These are typical components that make up a chilled water process loop. On the roof is an air-cooled chiller. The primary circulating pumps are located in a mechanical space below. The various floors indicate which equipment and systems are using this water for cooling purposes: walk-in refrigerators, future connections, computer room air conditioning units, and diagnostic equipment (i.e., MRI). Special water filtration is being provided to the MRI device.
The alternative to a separate process chiller is to use a plate and frame exchanger for “free” winter cooling from the main chiller (central plant) systems. This system ties into the condenser water system so that when the temperature outside is low enough, the building turns off its chillers and runs on the condenser water. The condenser water is allowed to flow through a cooling tower, through the heat exchanger, thereby producing useable chilled water on the other side of the HX which can be run through process systems. This “free” system still requires running pumps and motors for the cooling tower fan. A designer needs to weigh the advantages and disadvantages of these two systems before making a decision as to how to proceed.
Walk-in refrigerators and freezers offer a great opportunity to utilize a chilled water system, which can be much more efficient than a more standard refrigerant or DX based system. For individual rooms, the DX system still offers a significant first cost advantage. However, when six or more rooms are required and if the system requires redundancy for some or all of the cold rooms, this is when a chilled water system can be both more efficient and sometimes even lower in first cost. The designer must ask the manufacturer of the walk-in boxes whether a chilled water cooled system is available. If it is, then an analysis of the life-cycle costs will indicate if this potential energy saving design should be pursued.
The chilled water systems indicated here are all potential systems for a laboratory application. The main central systems are shown all the way to the individual laboratory systems for a piece of equipment. The system designer has now been shown some typical ways to apply chilled water equipment and has been shown choices with differences in efficiency and operation indicated.
Keith Cockerham, P.E., senior associate and project manager at CUH2A has 24 years of experience including all phases of mechanical systems design and engineering management. As Project Manager, Keith is a LEED® accredited professional working to promote sustainability through the design of buildings that are environmentally responsible, profitable and healthy. He has attained a Certificate in Energy Management (CEM) from the Association of Energy Engineers and is a Certified Building Commissioning Professional (CBCP). Keith is also an active member of other professional organizations including the American Society of Mechanical Engineers, and the American Society of Heating, Refrigerating, and Air Conditioning Engineers where he currently serves as a corresponding member of Technical Committee 9.10 Laboratories and where he has previously served as president of his local chapter. A graduate of Union College with a Bachelor of Science in Mechanical Engineering, Keith also earned an MBA from Rider University.