14-06-2012, 05:09 PM
HVAC AND REFRIGERATION SYSTEM
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Introduction
The Heating, Ventilation and Air Conditioning (HVAC) and refrigeration system transfers the
heat energy from or to the products, or building environment. Energy in form of electricity or
heat is used to power mechanical equipment designed to transfer heat from a colder, low-energy
level to a warmer, high-energy level.
Indoor air loop. In the leftmost loop, indoor air is driven by the supply air fan through a cooling
coil, where it transfers its heat to chilled water. The cool air then cools the building space.
– Chilled water loop. Driven by the chilled water pump, water returns from the cooling coil
to the chiller’s evaporator to be re-cooled.
– Refrigerant loop. Using a phase-change refrigerant, the chiller’s compressor pumps heat
from the chilled water to the condenser water.
– Condenser water loop. Water absorbs heat from the chiller’s condenser, and the condenser
water pump sends it to the cooling tower.
– Cooling tower loop. The cooling tower’s fan drives air across an open flow of the hot
condenser water, transferring the heat to the outdoors.
Vapour Compression Refrigeration
Heat flows naturally from a hot to a colder body. In refrigeration system the opposite must occur
i.e. heat flows from a cold to a hotter body. This is achieved by using a substance called a refrigerant,
which absorbs heat and hence boils or evaporates at a low pressure to form a gas. This
gas is then compressed to a higher pressure, such that it transfers the heat it has gained to ambient
air or water and turns back (condenses) into a liquid. In this way heat is absorbed, or
removed, from a low temperature source and transferred to a higher temperature source.
The refrigeration cycle can be broken down into the following stages (see Figure 4.2).
Absorption Refrigeration
The absorption chiller is a machine, which produces chilled water by using heat such as steam,
hot water, gas, oil etc. Chilled water is produced by the principle that liquid (refrigerant), which
evaporates at low temperature, absorbs heat from surrounding when it evaporates. Pure water
is used as refrigerant and lithium bromide solution is used as absorbent.
Common Refrigerants and Properties
A variety of refrigerants are used in vapor compression systems. The choice of fluid is determined
largely by the cooling temperature required. Commonly used refrigerants are in the family
of chlorinated fluorocarbons (CFCs, also called Freons): R-11, R-12, R-21, R-22 and R-502.
The properties of these refrigerants are summarized in Table 4.1 and the performance of these
refrigerants is given in Table 4.2.
Compressor Types and Application
For industrial use, open type systems (compressor and motor as separate units) are normally
used, though hermetic systems (motor and compressor in a sealed unit) also find service in
some low capacity applications. Hermetic systems are used in refrigerators, air conditioners,
and other low capacity applications. Industrial applications largely employ reciprocating, centrifugal
and, more recently, screw compressors, and scroll compressors. Water-cooled systems
are more efficient than air-cooled alternatives because the temperatures produced by refrigerant
condensation are lower with water than with air.
Selection of a Suitable Refrigeration System
A clear understanding of the cooling load to be met is the first and most important part of designing
/ selecting the components of a refrigeration system. Important factors to be considered in quantifying
the load are the actual cooling need, heat (cool) leaks, and internal heat sources (from all
heat generating equipment). Consideration should also be given to process changes and / or
changes in ambient conditions that might affect the load in the future.