Article - March 2002
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All-Variable Speed Centrifugal Chiller Plants: Can We Make Our Plants More Efficient?

Thomas Hartman, P.E.

Until a very short time ago, most chiller manufacturers made it very clear that variable speed did not belong in chiller plants. Adding VFDs to chillers and permitting water flow to vary were bad ideas. Now that has all changed. Nearly all manufacturers embrace the concept of chiller plants in which all the chillers, pumps and tower fans are variable speed. Such plants are called "all-variable speed" plants. Why the sudden change in attitude toward variable speed? The real question is why manufacturers have resisted the inevitable and beneficial integration of variable speed technologies into their chiller products for so long. Whatever the reasons, it's all history now, and designers or plant managers who employ chillers need to consider how to integrate these new technologies into their plants because they offer enormous cost reductions.

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Nearly all the major chiller manufacturers now offer complete lines of variable speed chillers, and for the most part they are hard at work retraining their regional and local representatives to talk a completely new talk of encouragement toward varying chilled and condenser water flow through chillers and operating these chillers at part loads where their efficiency is highest. Such ideas were heresy in the chiller industry just a short time ago, and like any substantial change, they are creating a lot of confusion. The purpose of this article is to give savvy designers and plant managers information needed to evaluate applying these new technologies to their cooling systems and to help them outline a plan for the most direct route toward implementing these technologies effectively where they show sufficient benefit.

A schematic of a typical all-variable speed chiller plant is shown in Figure 1 below. There are no differences in plant equipment (chillers, condenser pumps and cooling tower) layout between an all-variable speed chiller plant and a typical conventional plant. Although it is not shown, a header pumping arrangement on chilled and condenser water systems works equally as well with an all-variable speed chiller plant just as it does with constant speed plants. The real differences between a constant speed plant and an all-variable speed plant are the use of variable speed drives to operate all equipment and the DDC optimization control network that automatically coordinates the operation of the equipment under all operating conditions. Such automatic network control may or may not be employed in a conventional plant, but effective automatic control is a necessary ingredient for the success of an all-variable speed chiller plant.

Figure 1: All-Variable Speed Chiller Plant Configuration.
Figure 1: All-Variable Speed Chiller Plant Configuration.

Evaluating Benefits of All-Variable Speed Chiller Plants 

To evaluate this new approach to chiller plant design and operation, designers or plant managers must first get their arms around the potential benefits possible by building or converting their existing chiller plants to all-variable speed. Manufacturers believe that when properly applied, chillers in all-variable speed chiller plants will have longer useful lives and require less maintenance in their lifetime than constant speed chillers. However, at this point, there is not hard data to support such claims, and it is wise to limit a benefit analysis to energy savings alone. Figure 2 below shows the average annual operating efficiency for plants with electric motor driven centrifugal chillers, which make up the majority of larger chiller plants. Most such plants that keep their equipment updated and are well maintained operate in the "Fair" range from 0.85 to 1.0 kW/ton average annual efficiency. Those that also have effective efficiency monitoring and automatic optimized control systems may dip into in the "Good" range, but a conventional constant speed plant that shows an annual average operating efficiency much less than 0.8 kW/ton is a rarity, limited to those plants with exceptionally efficient equipment and good automatic optimization controls.

Figure 2: Chiller Plant Total Energy Use Spectrum
Figure 2: Chiller Plant Total Energy Use Spectrum

Now consider the range of annual efficiencies for an optimized all-variable speed chiller plant composed of the same basic equipment but operated entirely with network optimized variable speed control. Such all-variable speed plants generally operate at an average annual energy efficiency of 0.5 to 0.6 kW/ton, depending on the climate, the equipment employed, the configuration, and the load profile of the load served. This means that the potential reductions for converting a well maintained and operated chiller plant to an all-variable speed chiller plant can be expected to amount to between 25% and 50% or more of the current plant electric energy use. The resulting annual dollar savings amounts to a low of $30 to over $100 per installed ton, depending on the length of the cooling season, the load profile and the electric rate. For those considering absorption or engine driven chillers, consider that all-variable speed electric cooling will nearly always be vastly more economical to operate and much less costly to maintain. Unless waste heat is available, optimized all-variable speed chiller plant technologies make hybrid chiller plants obsolete. If demand limiting is mandated by rate structure, it can be accomplished far more effectively with a standby or emergency electric generator. Such an approach offers better economy and greater load shaping flexibility than incorporating non-electric chillers in the plant. 

All-Variable Speed for New Chiller Plants 

For new plants now in the planning stages the potential energy reductions make consideration of an optimized all-variable speed configuration an imperative. The marginal costs for incorporating optimized all-variable speed technologies into a new plant are very small. New variable speed chillers cost very little more than their constant speed alternatives. Applying variable speed drives to tower fans is cheaper than implementing two speed motors, and locating variable speed drives at the pumps (where they work best) can be less expensive than using regular starters housed in motor control centers. Payback on any extra investment required to configure a new plant as an all-variable speed chiller plant will usually be well under a year. 

Conversion of Existing Plants to All-Variable Speed 

Control Solutions, Inc Although the substantial energy reduction from all-variable speed technologies will nearly always justify incorporating all-variable speed into a new plant, it may not justify the costs of retrofitting an existing plant in good condition. For these plants, there are two paths to such a conversion that need to be considered. First, if the life cycle savings do justify it, an immediate retrofit involving adding variable speed drives to all the chillers, pumps and tower fans and implementing optimization controls should be considered. If not, a second path to consider is replacing chillers with variable speed chillers as each approaches the end of its useful life. In this path an upgrade of variable speed condenser pumps and tower fans along with network controls may be initiated immediately or as the new variable speed chillers come on-line.

The second approach is a fallback. It usually carries the same low marginal cost as that of planning a new plant because this path waits until the chillers reach the end of their useful life and have to be replaced anyway. If an immediate retrofit conversion to all-variable speed cannot be accomplished due to budget limitations, then the "conversion at time of replacement" should become the upgrade plan for the chiller plant.

Determining which upgrade path to follow requires an analysis of the savings potential of the conversion and the costs involved for an immediate retrofit. The most effective means of calculating the savings potential is to determine the present energy costs for the plant, estimate the present plant efficiency, and then apply the estimated efficiency improvement a conversion would bring. Such an evaluation can become more involved if the electric rate structure is dominated by a high demand charge. It may take some time to assemble the data necessary for this analysis, but it is not really complicated. It is also surprising how many resources are available to assist your efforts.

Once this energy savings estimate has been made, the next step is to estimate the costs involved in implementing a retrofit that will achieve the estimated savings. For plant owners or operators, the most direct route may be to call in manufacturers and contractors their company or institution has employed previously and with whom a good relationship has developed. With a frank discussion, and relying on more than a single source of pricing, it may be possible to get reliable budget pricing for adding the variable speed drives and necessary controls and making any other changes that may be necessary for a retrofit to all-variable speed operation. It should be noted that effective optimization control software modules are packaged technologies that are readily available at low cost. The packages include complete control sequences and the installation/operation support necessary to ensure the plant performs as expected. With some effort, it is not too difficult to arrive at a total upgrade cost estimate by assembling the cost of upgrading the chillers, pumps and tower fans to variable speed and adding a digital control system, if none presently exists, and the necessary control software and support.

Another approach is to have outside help to assist in this analysis. However, caution is advised. Because all-variable speed technology is relatively new, many engineers are not knowledgeable about all-variable speed plant operation, but few are eager to acknowledge this reality. It takes an experienced firm to do justice to such an analysis. Chiller manufacturers are now developing computer programs that can help with the savings portion of the analysis, and a manufacturer may be able to recommend a firm to assist with other aspects of the analysis. 

Implementing An Effective All-Variable Speed Chiller Plant 

Whether the all-variable speed chiller plant is a new one under design or the result of a decision to retrofit an existing plant, achieving a successful result requires close attention and cooperation by those responsible for the design and operation of the plant. To be successful, an all-variable speed chiller plant requires a method of automatic control that optimizes the equipment operation under all loading conditions.

Unlike constant speed plants in which the plant generally achieves peak efficiency when chillers are fully loaded, an all-variable speed chiller plant operates most efficiently when chillers, pumps and tower fans are all operating at partially loaded conditions. An operating efficiency graph for a conventional constant speed chiller plant and an optimized all-variable speed centrifugal chiller plant (each consisting of just one chiller) is shown below in Figure 3. One can easily adopt this graph to estimate the energy use of a multiple-chiller plant at any capacity and wet bulb condition simply by dividing the total load served by the number chillers in the plant that would be on-line at various wet bulb and capacity conditions. Note that when dealing with multiple-chiller plants, all-variable speed plants benefit from sizing the chillers all the same.

Figure 3 shows that at all conditions except when operating at high outdoor wet bulb temperatures and high capacity conditions, a properly optimized all-variable speed chiller plant will operate more efficiently than a constant speed plant with chillers, pumps and towers of the same nominal efficiencies. Chiller plants that have at least 20% excess capacity for redundancy can operate at better efficiencies at peak loads than constant speed plants. Maintaining some redundant capacity in an all-variable speed plant provides not only protection against equipment failure but also improvements in plant efficiency at peak load conditions. These are the major differences between constant and all-variable speed chiller plants. Beyond them, all-variable speed plants are generally configured the same as constant speed plants, making laying out new plants or retrofitting many existing plants quite straightforward.

Figure 3: Comparison of Constant Speed (dashed lines) and Optimized All-Variable Speed Chiller Plant average operating efficiencies at various wet bulb and plant capacity conditions

An item worth noting by facility managers is that variable speed equipment should be designed for 480 volt operation, and generally, 1500 to 2000 tons is an optimum size for variable speed chillers in terms of cost effectiveness. Higher voltages or larger sized variable speed drives are usually cost prohibitive. If an existing plant employs a higher voltage or larger sized chillers, is it most likely that an immediate retrofit will not be cost effective. However, it is also important to note that chillers in sizes greater than 2000 tons are continually becoming more costly to purchase, so that at replacement time plant operators should plan on employing multiple chillers to replace larger existing machines. At that time a switch to lower voltage can be made along with the conversion to variable speed.

Converting to an all-variable speed chiller plant need not affect the chilled water distribution system. Older primary/secondary distribution systems can be employed, but it is highly recommended to consider upgrading such a system to a single circuit variable flow system (sometimes called a variable primary flow system) as a part of the upgrade plan. When the chilled water distribution system serves a large complex or campus, it may be advantageous to employ pumps in each building. In a single circuit variable flow distribution system these are direct coupled in series with the primary pumps without any bypass, thereby operating as "booster" pumps. Network control enables the coordination between the primary and booster pumps to affect significant reductions in distribution pumping energy. Packaged software is also available for optimizing primary-booster pumping, just as it is for optimizing the all-variable speed chiller plant, and this software can easily be designed to absolutely end the problems associated with low delta T in the distribution system. This packaged software also greatly simplifies startup and ensures performance criteria will be met. 

Operating Considerations For All-Variable Speed Chiller Plants 

As stated earlier, the operational differences between a conventional constant speed chiller plant and an all-variable speed chiller plant are considerable. To obtain optimum efficiency of the plant, the variable speed chillers, pumps and towers must be sequenced so that they operate at partial capacity whenever possible. The only time any equipment should be operated at full capacity is when all plant equipment must operate at full capacity to meet a peak load. When a plant is oversized for redundancy, this will only be necessary in the event of a component failure. Although this means more equipment is generally on line at any given time, manufacturers and operators agree that the maintenance of the equipment is reduced in all-variable speed chiller plant configurations. The reasons for maintenance reductions are softer and less frequent starting and stopping and lower average loading of the equipment. These factors more than compensate for the longer operating hours.

Plant managers and operators should consider that the only method of effectively optimizing the operation of an all-variable speed chiller plant is to employ automatic controls to sequence chillers and coordinate the speed of the condenser water pumps and tower fans. Currently, many large plants are operated manually or semi-manually. A change to automatic control requires an effective optimization control package that gives the operations staff both the insight and oversight required to ensure the plant operates smoothly throughout all conceivable conditions. It also requires monitoring to let the operations staff know at all times if the plant is meeting its performance criteria for the current operating conditions. For a staff that is used to manually operating the chiller plant equipment, this may require special training. However, once the operations staff understands their new (and more important) role in the operation of an automatically operated plant whose performance they constantly monitor and evaluate to make a measurable contribution to the facility, they will almost always be pleased with their new role. 

Summary And Conclusion 

All-variable speed chiller plants employ relatively new technologies that have the capacity to reduce cooling energy use by 25% to more than 50% below well operated constant speed plants. Plant designers and managers need to consider this new approach to configuring and operating chiller plants as they plan new plants or consider upgrading existing ones. Managers and operating staff also need to understand the operations and maintenance implications of this technology which rightfully will challenge traditional perspectives. In order to achieve the ultra-efficient chiller plant operations it promises, new perspectives on chillers, towers, condenser flow, and plant operations are required. However, this is a fundamentally straightforward and easily applied technology. It may take some time to upgrade existing plants, but the results will be well worth the effort.

Be sure to read Tom's follow up April article All-Variable Speed Chilled Water Distribution Systems: Optimizing Distribution Efficiency 

Reference List: Other sources of information on this subject by the author


ALL-VARIABLE SPEED CHILLER PLANTS, ASHRAE JOURNAL, September, 2001. (you need to be an ASHRAE Member to Access this article on line)




Additional information on technologies discussed in this article is available at  Any questions or comments about the article may be addressed to Mr. Hartman at

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