Although centrifugal chillers may have lower operating costs, potable water for cooling towers may be scarce. In such cases, other sources like sea, lake, river or pond water, or even treated sewage effluent may be used. However, these sources may require more expensive construction materials, corrosion protection, etc., which result in lower heat exchanger efficiency. This will offset some of the energy efficiency benefits of the water-cooled centrifugal chiller system. The higher cost of water treatment and corrosion protection could favor an air-cooled chiller system.
Many of the city municipalities charge for sewage disposal, which may need to be accounted for considering the bleed off from the cooling tower based on the cycle of concentration with a consequent impact the payback period for water-cooled chillers.
If water availability may be a challenge, stakeholders that face this issue may consider a hybrid system. Hybrid systems combine air-cooled and water-cooled chillers in different ratios of 30/70 or 40/60 or 50/50 based on the load profile, storage capacity and critical base demand, etc. The hybrid approach can mitigate future risks and optimize present resources. Desert and hilly regions are particularly challenged, but so are major cities that are unable to meet the potable water demands and are subject to strict guidelines and regulations. Receding ground water level can cause municipal authorities to stop ground water drilling for commercial use. Under such uncertainties, building a mission critical facility with a water-cooled chiller is unrealistic.
Repair and maintenance factors are difficult to assess. These costs depend on complex variables, including the quality of air and water, size and complexity of plant design, quality and make of equipment, location and distance from nearest service center, and labor cost per hour for skilled manpower in a particular city. The easiest assessment is based on an inventory of the number and types of equipment that will require maintenance. Although repair and maintenance costs are more subjective than factors with an immediate cost, they should be considered based on the experience of the designer/user. Also, new chiller technologies can be factored in that use magnetic-bearing centrifugal compressors, which can reduce maintenance costs and increase chiller life.
Demand or connected load charges are normally higher for air-cooled chiller plants due to higher design peak kW load requirements. These costs can be considered while performing an energy cost comparison. Electrical equipment and system costs are based on total connected load, which is higher in an air-cooled plant and may impact overall budget of a building’s utilities. With a higher connected load requirement for an air-cooled chiller and a higher associated electrical system cost, overall capital cost of both systems may be very close.
Space availability and cost is a major concern for builders in large cities, where real estate costs can be as high as $4,000 per square foot. Water-cooled chillers are normally housed in an enclosed space within the building or a separate building nearby. This lost space is an opportunity loss for the builder who could otherwise sell or rent out the floor area. In contrast, air-cooled chillers are normally kept on the roof of the building, not occupying valuable saleable space. Where space cost is not at a premium, space cost is not a factor. Building construction cost may be relatively small or go substantially higher if local codes require extra load bearing capacity for an air-cooled chiller or a seismic design requirement of the site. In this regard, an air-cooled chiller has an advantage, because it can be kept on the roof without any change in roof construction cost. However, if a water-cooled chiller must be kept on the roof for floor space, not only the load bearing capacity of the roof should be increased due to concentrated load, the equipment must also be modified to withstand ambient conditions outdoors.
For more information, contact Chicago Compressor & Specialty Products.