Chilled Water system

Transcription

1 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. 1

2 Chiller Fans Heat Rejection 3 Condenser 2 Expansion valve Refrigerant Gas Compressor Refrigerant Liquid + Vapour 4 Heat Input Evaporator 1 Chilled Water flow Chilled Water return Pump - Control Valve Cooling Coil(s) Chiller Schematic Diagram with Air Cooled Condenser 2

3 Condenser Water Cooling Tower Chiller Heat Rejection Expansion valve 3 Condenser 2 Refrigerant Gas Compressor Refrigerant Liquid + Vapour 4 Heat Input Evaporator 1 Chilled Water flow Chilled Water return Pump - Control Valve Cooling Coil(s) Chiller Schematic Diagram with Cooling Tower 3

4 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. Expansion valve 3 Fans Heat Rejection Condenser 2 Refrigerant Gas Compressor Air Cooled Condenser incorporating Compressor(s) Refrigerant Liquid + Vapour 4 1 Refrigerant Flow Refrigerant Return Heat Input - Direct Expansion (DX) System Direct Expansion Cooling Coil also known as DX coil in Air Handling Unit (AHU) 4

5 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 4.0 o C and 8.0 o C. 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 wellinsulated. 5

6 Chilled Water Temperatures Pipe Design Lecture No.(2) By Badran M. Salem Typically chilled water flow and return temperatures to cooling coils is generally between 7 o C and 12 o C, 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 o C to 6 o C. 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. 6

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