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HVAC Design Summary

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  CHW Plant Design Guide Summary COMPRESSORS    Reciprocating    Rotary Types of Rotary in HVAC Industry a.   Scroll b.   Single blade (fixed vane) c.   Rotating vane d.   Screw Two Types in used today a.   Single screw b.   Twin screw    Centrifugal Surge - is a condition that occurs when the compressor is required to produce high lift at low volumetric flow, During a surge condition, the refrigerant alternately moves backward and forward  through the compressor, creating a great deal of noise, vibration, and heat . Prolonged operation of the machine in surge condition can lead to failure . Easy to detect in that the electrical current to the compressor will alternately increase and decrease  with the changing refrigerant flow   Incipient surge  - which the machine gurgles and churns , this is not harmful  to the compressor but may create unwanted vibration. The electrical current does not vary  during incipient surge. The capacity of centrifugal compressors may be controlled by three methods:    The most common is to use inlet guide vanes or prerotation vane.    vary the speed of the impeller in conjunction with using inlet guide vanes    hot gas bypass (HGBP) CHILLERS Two types of evaporators are used in water chillers:    flooded shell and tube    the direct expansion evaporators (DX)    Flooded Shell and Tube    The flooded shell and tube heat exchanger has the cooled fluid (usually water) inside the tubes and the refrigerant on the shell side (outside the tubes).   Direct Expansion    The direct expansion (DX) evaporator has the refrigerant inside the tubes and the cooled fluid (usually water) on the shell side (outside the tubes) .  ARI 550/590-2003 and 560-2000 Rating Conditions for Water Chillers IPLV  - “integrated part - load value” . Part-load efficiency for equipment is based on the weighted operation at various load capacities for the equipment. The equipment COP is derived for 100%, 75%, 50%, and 25% loads  (with consideration for condenser water relief) and is based on a weighted percentage of operational hours (assumed) at each condition  Reciprocating and Scroll Chillers    These chillers are widely used in tonnage ranges from 50 to 230 tons  although they are available up to much larger sizes (400 tons and up). Most frequently the compressor design is semi-hermetic compressors Rotary Screw Chillers    Screw chillers tend to be most cost competitive in the 100 ton to 300 ton range  although they are available in a wider range of capacities.    Screw chillers tend to be very noisy at design conditions due to the high speed of operation . Centrifugal Chillers    Centrifugal chillers have the highest efficiency ratings  of all the chillers discussed. They are available in sizes from 8 0 tons to 10,000 tons but the most common sizes are from 200 to 2,000 tons    In addition to the energy savings, centrifugal chillers with variable speed drives are quieter at part load  and are likely to last longer. Absorption Chillers      Sizes for absorption chillers range from 100 to 1,700 tons . Absorption machines typically cost two or more times that of an electric-driven chiller Turbine-Driven and Engine-Driven Chillers    Both use the same vapor compression cycle as an electric machine except it uses a reciprocating engine, or a gas- or steam-driven turbine as the prime mover COOLING TOWERS Range  - The temperature difference between the water entering  the cooling tower and the temperature leaving the tower . Approach  - The temperature difference between the water leaving  the cooling tower and the ambient wet-bulb temperature . Types of Cooling Towers    Spray Towers - Spray towers distribute high-pressure water through nozzles into a chamber where air is induced to flow with the water spray. There are no fans, Spray towers are seldom used.       Forced Draft Cooling Towers  - Forced draft towers can be of the crossflow or counterflow type, with axial or centrifugal fans.    Induced Draft Cooling Towers  - The induced draft tower is by far the most widely used and energy-efficient cooling tower available in the HVAC industry   Application Issues    Siting and Recirculation - When the saturated air leaving the cooling tower is drawn back into the intake of the tower, the recirculation that occurs degrades the performance of the tower    Capacity Control a.   On/Off - Cycling fans is a viable method but leads to increased wear on belts and drives (if used) and can lead to premature motor failure b.   Two-Speed Motors  - Multiple wound motors or reduced voltage starters can be used to change the speed of the fan for capacity control. This method is cost effective and well proven c.   Pony Motors  - This is another version of the two-speed approach. A second, smaller motor is belted to the fan shaft. d.   Variable-Speed Drive (VSD - Adjustable frequency drives can be added to the motors for speed control. This method provides the best temperature control performance and is the most energy-efficient method of control .    Chemical Treatment and Cleaning . a.   Cooling towers are notorious for requiring high maintenance
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