Chilled Water system - Donutsdocshare01.docshare.tips/files/17552/175529404.pdf · Chilled Water...
Transcript of Chilled Water system - Donutsdocshare01.docshare.tips/files/17552/175529404.pdf · Chilled Water...
Pipe Design Lecture No.(2) By Badran M. Salem
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Chilled Water system
Introduction
For large installations the Condenser, Evaporator, Compressor and
Expansion device can be purchased as a package unit, known as a
Chiller . The usual package consists of electrically driven
compressor(s) mounted on top of two shell and tube heat exchangers,
one for the evaporator and the other for the condenser.
The cooling coil(s) are piped up to the chiller in the conventional
manner as shown below.
In some countries the Cooling tower is the preferred method of removing heat from the system. Cooling towers that are open to atmosphere are not often used since the water may become contaminated. A closed cycle cooling tower or evaporative cooler can be used to
reduce contamination risk.
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Chiller Schematic Diagram with Air Cooled Condenser
Cooling Coil(s)
Condenser
Evaporator
Compressor
Heat Rejection
Heat Input
Expansion
valve
1
2
3
4
Refrigerant Gas
Refrigerant Liquid
+ Vapour
-
Pump
Control
Valve
Chilled
Water
flow
Chilled
Water
return
Fans
Chiller
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Cooling Coil(s)
Condenser
Evaporator
Compressor
Heat Rejection
Heat Input
Expansion
valve
1
2
3
4
Refrigerant Gas
Refrigerant Liquid
+ Vapour
-
Pump
Control
Valve
Chilled
Water
flow
Chilled
Water
return
Chiller
Cooling Tower
Condenser Water
Chiller Schematic Diagram with Cooling Tower
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Alternative arrangements are shown for smaller installations where the condenser may be mounted on the roof or external wall of a building and cooled by outside air. The evaporator may be installed directly into the ductwork or air handling unit (AHU) for smaller installations. This is known as a Direct Expansion (or DX) coil. A typical Direct Expansion (or DX) system is shown below. This avoids using condenser water and chilled water in the system and installing the accompanying plant.
Direct Expansion Cooling Coil
also known as DX coil in Air
Handling Unit (AHU)
Condenser
Compressor
Heat Rejection
Heat Input
Expansion
valve
1
2
3
4
Refrigerant
Gas
Refrigerant Liquid +
Vapour
-
Refrigerant
Flow
Refrigerant
Return
Fans
Air Cooled
Condenser
incorporating
Compressor(s)
Direct Expansion (DX) System
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Chilled-water System
In larger buildings and particularly in multi-story buildings, the split-system
approach begins to run into problems. Either running the pipe between the condenser and the air handler exceeds
distance limitations (runs that are too long start to cause lubrication difficulties in the compressor), or the amount of duct work and the length of ducts becomes
unmanageable. At this point, it is time to think about a chilled-water system.
In a chilled-water system, the entire air conditioner is situated on the roof or behind the building.
It cools water to between 4.0oC and 8.0
oC.
This chilled water is then piped throughout the building and connected to the
cooling coils in air handlers as needed. There is no practical limit to the length of a chilled-water pipe if it is well-insulated.
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Chilled Water Temperatures
Typically chilled water flow and return temperatures to cooling coils is generally between 7
oC and 12
oC, depending upon the dew point to be maintained.
When this water is pumped through the evaporator section of the chiller this water temperature will be lowered by about 4
oC to 6
oC.
In order that the necessary heat transfer may take place, the refrigerant must be at some temperature below that of the leaving water but, at the same time, it must
generally be slightly above freezing point.
In a typical case, the following water temperatures may be used:
Apparatus dew point 12°C Cooling coil outlet 10°C
Cooling coil inlet 6°C Water at evaporator outlet 5.5°C
The refrigerant in the evaporator would in this case be maintained at about 1°C
giving a differential for 4.5°C for heat transfer. As will be appreciated, this small temperature potential means that the cooling surface of a simple tubular type would need to be very extensive: a variety of
devices has been developed to augment the transfer rate.
Ethylene/glycol solutions may be used in cooling coils in order to allow lower air temperatures to be obtained.
The temperatures of the fluid circulating may be -7°C from the evaporator and -3°C returning to it, or lower as required.
In instances where cooling for an air-conditioning system is provided from a refrigeration machine by direct expansion, the refrigerant is piped directly to
cooling coils in the air stream which thus become the evaporator.
The surface temperature of the coils is a function of the leaving air temperature required, the form of the coil surface and the velocity of the air flow. Refrigerant temperatures much below freezing point are inadmissible owing to the
risk of build-up of ice on the coil surface when dehumidification is taking place. An apparatus dew point of 3°C is normally considered as the practical minimum for
such coils if frosting is to be avoided.
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