Since the beginning of HVAC time, system designers tried to select heating and cooling equipment that is the right size for the job. Often they succeed; sometimes they do not. Even the most careful designer can oversize or undersize an HVAC system. The approach and guidelines for correctly sizing the chiller is an involved issue and will be left to more daring authors. The goal of this article is to better inform the reader on what to look for in a chiller to increase the chances of continued operation in the event the chiller is oversizing.
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When a chiller is undersized, problems can appear immediately; the space is not held to the correct temperature and the tenants complain. For an oversized set-up, the problems are not as obvious...at first. Comfortable occupants and a system that operates do not always mean that all is well. During low load conditions, the unit may be starting and stopping on a regular basis. If oversized, the unit will match the load, turn off, and turn back on when there is another call for cooling. This cycling can be detrimental to the mechanical components of the unit.
Compressor and motor
Compressors do not like the pains and stresses that starting can cause. The problem is wear. When a chiller starts, the motor slams into action, so to speak. This, slamming causes wear on the gears and other moving parts in the motor. A direct-drive motor has no gears and, therefore, no gear slamming.
Selecting a screw chiller for a job that has a direct-drive compressor is an example of using the old adage "An ounce of prevention." When a direct-drive compressor starts, all of the force is transferred to the large surface area of the rotors instead of the small surfaces that gears represent. Very rarely do you hear of a compressor failure attributable to rotor wear.
To reduce this slamming force, gear-driven compressors are often wired with a reduced inrush starter--a legitimate way to reduce gear wear. Both solid state and wye-delta (star-delta) starters gradually increase the current supplied to the compressor so that the force on the gears is spread over time. The problem is that these types of starters add cost and complication to the system. Some chiller manufacturers require condenser water to cool the silicon control rectifiers on the starter. This coolant adds complexity and an extra pump to the starter system as well as requiring water to come in close proximity to medium voltage electricity.
A direct-drive compressor does not require reduced inrush starting. With these chillers, the only legitimate reasons for a solid state, wye-delta or delta-delta starter would be factors external to the unit itself. If the electrical service to the chiller requires a reduced inrush starter, the wye-delta is typically less expensive than a solid state unit and has a greater reduction of inrush current (66 percent) than either delta-delta (40 percent) or solid state (50 percent).
If the electrical environment in which the chiller is installed requires a reduced inrush starter, then it's important to select a chiller that disables the starter after the chiller is running. Many manufacturers use starters that remain powered, even after start-up. The continuous application of power causes unnecessary wear on the silicon control rectifiers; it is a case similar to leaving a car key turned to the start position. Set up your chiller to have the incoming power bypass the starter while the unit is running.
Another problem encountered on oversized screw chillers is long anti-recycle times. When there is no longer a demand for cooling, the unit shuts off. If the need for chilled water arises between preset stop and start cycles, the unit will not start. The unit's operation no longer matches the load and this forces the chiller to work harder to catch up when it does restart. Choosing a chiller with short stop and start times ensures that the load will be more closely matched, even when the unit is oversized.
Smart controls, in addition to short anti-recycle times, more closely match the load on an oversized unit. Smart controls allow the chiller to operate with low evaporator pressure, high condensing pressure, and motor over-current conditions. An oversized chiller application may present problems with low evaporator pressure. A smart control unloads the compressors when it detects low evaporator pressures and this allows the unit to continue operating. This feature, along with the short anti-recycle times, helps the oversized chiller more closely match the load.
More on motors
Screw chillers with motors running at high speed or with open drive motors have long anti-recycle times. Gear-driven compressors running at high speed need more time between starts because the air around the unit cools the open drive motor. This air is sometimes at a high temperature and is always less efficient at cooling motors than either liquid or vapor refrigerant.
Following these suggestions when choosing a screw chiller helps ensure continued operation of the HVAC system. However, selecting a unit this way does not mean that intentionally installing a 400-ton chiller on a 300-ton application will not cause problems. The chiller will start and stop constantly during low load conditions and this is not healthy for any unit. Choosing a chiller with the features described above means that the unit should continue operating and would closely match the load, even if it were oversized.