Chillers

A chiller is a liquid chilling system that cools a secondary coolant (usually water) for centralized air conditioning systems. The basic components of a vapor-compression liquid chilling system include a compressor, a liquid cooler (evaporator), a condenser, a liquid refrigerant expansion or flow control device and a control center.

Principles of operation
Water enters the evaporator, where it is chilled by liquid refrigerant evaporating at a lower temperature. The refrigerant vaporizes and is drawn into the compressor, which increases the pressure and temperature of the gas so that it may be condensed at the higher temperature in the condenser. The condenser cooling medium is warmed in the process. The condensed liquid refrigerant then flows back to the evaporator through an expansion device.
Chilled water enters the evaporator at 55 ºF (12.8 ºC), for example, and leave at 45 ºF (7.2 ºC). Condenser water leaves a cooling tower at 86 ºF (30 ºC), enters the condenser, and returns to the cooling tower near 95 ºF (35 ºC). Water cooled chillers are generally located within the building and use cooling towers located outside the building to reject the heat . Condensers may also be cooled by air or through evaporation of water. Air cooled chillers are generally located outside the building and reject heat directly to the atmosphere. A systems with a single compressor and one refrigerant circuit with a water-cooled condenser, is used extensively to chill water for air conditioning because it is relatively simple and compact .



	Fig. Nº80. Typical Water-Cooled Chiller Unit.
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	Fig. Nº81. Typical Air-Cooled Chiller Unit.
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	Fig. Nº82. CBasic Diagram for a Liquid Chiller.
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Multiple Chiller systems

A multiple chiller system has two or more chillers connected by parallel or series piping to a common distribution system. Multiple chillers offer operational flexibility, standby capacity, and less disruptive maintenance. The chillers can be sized to handle a base load and increments of a variable load to allow each chiller to operate at its most efficient point. Multiple chiller systems offer some standby capacity if repair work must be done on one chiller. Starting in-rush current is reduced, as well as power costs at partial load conditions. Maintenance can be scheduled for one chilling machine during part load times, and sufficient cooling can still be provided by the remaining unit(s). However, these advantages require an increase in stalling costs and space.

Two basic multiple chiller systems are used: parallel and series chilled water flow. In the parallel arrangement, liquid to be chilled is divided among the liquid chillers; the multiple chilled streams are combined again in a common line after chilling. As the cooling load decreases, one unit may be shut down. Unless water flow is stopped through the inoperative chiller, the remaining unit(s) provide colder than design chilled liquid. The combined streams (included one from the idle chiller) then supply the chilled water at the design temperature in the common line. All units should be controlled by the combined leaving water temperature or by return water temperature. Chilled water temperature can be used to cycle one unit off when it drops below a capacity that can be matched by the remaining units.

In the series arrangement, no overchilling by either unit is required, and compressor power consumption is lower than it is for the parallel arrangement at partial loads. However, the chilled water pressure drop may be higher if shells with fewer liquid side-passes or baffles are not available. Water cooled condensers in series are best piped in counter flow arrangement so that the lead machine is provided with warmer condenser and chilled water and the lag machine receives colder entering condenser and chilled water. This way refrigerant compression is nearly the same for each unit and identical units can be used. Thus, either machine can provide the same standby capacity if the other is down, and lead and lag machines may be interchanged to equalize the number of operating hours on each.

In both, series or parallel arrangement, one unit should be shut down as soon as possible with the remaining unit(s) carrying the full load. This not only reduces the number of operating hours on a unit, but also means less total power consumption because the COP tends to decrease below the full load value when unit load drops much below 50 %.

Chiller Types

Different types of chiller are used depending upon the type of compressor used as part of the refrigeration circuit. The different types of compressor are as follows:

1.- Reciprocating:
Reciprocating chillers are more common and in general are cheaper than other compressor types, particularly for the smaller sized units. Reciprocating compressors are used from smaller unit sizes up to around 450 Ton. They have many moving parts and this can increase maintenance costs. A reciprocating chiller is distinguished from centrifugal and screw compressor-operated chillers by its use of increments of capacity reduction rather than continuous modulation. Reciprocating chillers are available with simple on-ff cycling control in small capacities and with multiple steps of unloading down to 12.5 % in the largest multiple compressor units.




	Fig. Nº83. Reciprocating Air-Cooled Chiller Unit and Reciprocating Compressor
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Most intermediate sizes provide unloading to 50, 33, or 25 % capacity. Although relatively inefficient, hot-gas bypass can be used to reduce capacity to nearly 0 % with the unit in operation. However, part load performance can be improved using variable speed control.

Rotary Scroll:
Scroll compressors are used in smaller units (up to 80 Ton) being quiet and smooth-operating units with the highest efficiency ratio of all compressor types. They are currently used in residential and commercial air-conditioning. Most scroll compressors used today are of the hermetic type, which require virtually no maintenance.

Rotary Screw:
Screw compressors are used in the mid-range of unit sizes, from around 25 Ton up to 800 Ton. They are compact and have less moving parts, hence lower maintenance costs and longer lifespans. Screw compressor liquid chillers include a slide vane that adjusts the length of the compression path. Inverter-driven, variable-speed electric motors and turbine and engine drives can also modulate screw compressor speed to control capacity. Most screw compressors chillers modulate from 100 % to approximately 10 % load. Hot-gas bypass can be used to reduce capacity to nearly 0 %.


Fig. Nº84 Rotary Scroll Air-Cooled Chiller Unit and Rotary Scroll Compressor .
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Fig. Nº85 Rotary Screw Water-Cooled Chiller Unit and Rotary Screw Compressor .
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Fig. Nº86 Centrifugal Water-Cooled Chiller Unit .
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Rotary Centrifugal:
Unlike reciprocating and rotary screw, centrifugal compressor is not a constant displacement machine, thus it offers a wide range of capacities continuously modulated over a limited range of pressure ratios. They are well suited to compressing large volumes of refrigerant to relatively low pressures. By altering built-in design items as number of stages, compressor speed, impeller diameter, and choice of refrigerant, it can be used in chillers having a wide range of design chilled liquid temperatures and design cooling fluid temperatures.

Its ability to vary capacity continuously to match a wide range of load conditions with nearly proportional changes in power consumption makes it desirable for both close temperature control and energy conservation. Its ability to operate at greatly reduced capacity allows it to run most of the time with infrequent starting.

Centrifugal packages are currently available from about 80 to 2400 Ton (upper limit continuously increasing) at nominal conditions of 44 ºF (6.7 ºC) leaving chilled water temperature and 95 ºF (35 ºC) leaving condenser water temperature. Efficiencies at full load are high, but partial load control will only achieve reduced performance unless control capacity by speed regulation is applied. Centrifugal compressors may be driven by steam turbines, gas turbines or engines; however electric motor-driven machines constitute the majority of units sold.