Engineers can employ an ever-increasing array of technologies to meet the cooling needs of any process. Designers often conduct a benefit-and-risk evaluation when pondering which technology to em-ploy. An abundant quantity of sales material trumpets the benefits of each technology, but realistically evaluating the risks can be a more challenging task.
The most often employed technology for cooling is the vapor compression cycle. The powerhouse of the vapor compression cycle is the compressor. The four most commonly applied compressor technologies used with chillers are reciprocating, scroll, helirotor (screw) and centrifugal.
The greatest financial risk to a process is downtime due to unscheduled service or catastrophic equipment failure. Each compressor technology requires a different approach to dealing with downtime: repair, replace, reliability, rental or resilience.
RepairLarge reciprocating compressors are designed and manufactured to reduce the expense and downtime caused by compressor failure by accommodating convenient repair access. Compressor de-signs can be open or hermetic. Open indicates the electric motor that drives the compressor is outside the compressor casing; hermetic indicates that the motor is inside the compressor casing. With reciprocating compressors, semihermetic defines a hermetic compressor casing that can be opened easily for repair.
The strenuous, unrelenting up-and-down motion of a reciprocating compressor causes wear. Reciprocating compressors require inspection and possibly repair after 7,000 to 20,000 hr of operation, depending on application severity. Semi-hermetic compressors are designed to facilitate repair at the job site. This compressor type also can be removed from the job site and repaired where trained technicians have ready access to appropriate tools and OEM parts.
Contractor-repaired compressors commonly are called rebuilt compressors. Many service contractors maintain a small inventory of these units to allow them to quickly replace failed compressors, reducing downtime.
Alternatively, failed semihermetic compressors can be remanufactured: Compressors are returned to the manufacturer and completely disassembled. Only recycled parts that meet manufacturer's specifications are used for remanufacture. Therefore, remanufactured compressors usually carry a full manufacturer's warranty.
ReplaceScroll compressors have fewer moving parts than reciprocating compressors, and their lower part count improves compressor reliability and reduces assembled cost. In addition, the low cost of computer-aided manufacturing creates an environment where replacing a scroll compressor is less expensive than the labor involved in repair. Thus, response to scroll compressor failure is simply to replace it. Complete compressor replacement also is a possible response to downtime of smaller hermetic reciprocating compressors.
Unlike reciprocating compressors, which respond to load variations by loading or unloading cylinders, scroll compressors simply cycle on and off. They are employed in multiples to provide the needed part-load capability.
A 60-ton water chiller can employ a single 60-ton semihermetic reciprocating compressor with unloading cylinders. Alternatively, the same chiller can employ four 15-ton scroll compressors manifolded into two independent refrigerant circuits. These design differences impact chiller reliability.
A failure of the single reciprocating compressor completely incapacitates the chiller. When designed with two independent refrigerant circuits, even with a failed scroll compressor, the chiller continues to provide 30 tons of capacity.
While dual refrigerant circuits provide continued, yet diminished, capacity in the wake of a compressor failure, multiple compressors put an added burden of reliability on scroll compressors. Whether repairing a reciprocating compressor or simply replacing a scroll compressor, any compressor failure reduces process cooling capacity and negatively impacts maintenance budgets.
By employing multiple scroll compressors to replace a single reciprocating compressor, the number of compressors required to meet capacity is greater. Scroll compressors must have a higher degree of reliability if the owner of a cooling process is to experience fewer compressor failures. Although most scroll compressors are not repaired, rebuilt or remanufactured, failed scroll compressors often are returned to the manufacturer to determine the cause of failure. Lessons learned from this analysis allow manufacturers to improve compressor reliability.
ReliabilityReliability of centrifugal or helirotor compressors is enhanced by intelligent unit-level controls. Even in trying situations, intelligent controls can keep the chiller online. Instead of turning the chiller off, adaptive controls increase or decrease chiller capacity to less-threatening operating conditions. Further, unit controls will communicate impaired operating conditions to system-level controls and the process operator. For example, intelligent chiller controls can request cooler or warmer water from the cooling tower.
Statistically, centrifugal compressor failure is near zero, so the chance of failure or downtime is low. However, the process cooling designer must evaluate the liability of a helirotor or centrifugal compressor failure. If the downtime liability is unaffordable, backup equipment is required. Such a design concept is expressed as N+1 or N+2. N+1 means the process cooling systems will meet any cooling requirement even after one pump, compressor or cooling tower has failed. N+2 designs will meet all cooling requirements with up to two compressors failed. N+1 or N+2 can be an expensive way to achieve absolute reliability - the additional pumps, chillers and cooling towers represent large capital assets that are idle.
RentalAn alternative to committing large capital assets to idle equipment in an N+1 or N+2 strategy is N+R, where R stands for rental. Many service contractors and manufacturers maintain a fleet of rental chillers pre-engineered for rapid deployment. Often, the rental units are trailer mounted with onboard pumping capability.
Downtime can be minimized with an emergency recovery plan. If the electrical connections and chilled water supply and return stubouts are designed to be accessible from a temporary chiller location, the rental chiller can be producing chilled water just hours after reaching the job site.
ResilienceReciprocating, scroll and helirotor compressors all are defined as positive- displacement compressors. In these de-signs, the volume of vapor compressed by a piston stroke is relatively constant. By contrast, centrifugal compressors are true variable-volume devices. Vanes at the compressor inlet meter the volume of vapor to be compressed. The ability to vary volumetric capacity allows centrifugal chillers to operate over a range of conditions.
Centrifugal compressors reach peak efficiency near 70 to 80% of maximum capacity. This leaves most centrifugal chillers with a degree of reserve capacity. Incorporating this additional capacity into the design of a large process cooling load creates a measure of system resilience. If one centrifugal chiller is unavailable, the remaining centrifugal chillers tap into reserve capacity. If a shortage of chilled water pumps occurs, centrifugal chillers can produce colder water to meet the load. Likewise, if a tower capacity shortage occurs, centrifugal chillers can produce warmer condenser water resulting in an increase of tower capacity from a limited number of cooling tower cells.
Chillers rely on the operation of the compressor. The type of compressor selected will affect how downtime is avoided or reduced, and at what cost. If the Rs of refrigeration - repair, re-place, reliability, rental and resilience - are employed effectively in the design and operation of a process cooling system, the last R - regret - can be avoided.