8/24/2015

helical-rotary water chiller Refrigeration Cycle

helical-rotary water chiller
Refrigeration Cycle




Refrigeration Cycle:

A pressure-enthalpy (p-h) chart illustrates the refrigeration cycle of the helical- rotary water chiller.


Refrigeration Cycle chiller
But first, lets review the components of the helical-rotary chiller’s refrigeration cycle...
Refrigerant vapor leaves the evaporator and flows to the compressor where it is compressed to a higher pressure and temperature. Oil is removed from refrigerant vapor in the oil separator and the refrigerant travels to the condenser while the oil is recirculated back to the compressor.
In the condenser, the refrigerant vapor rejects heat to water or air and leaves as a sub-cooled liquid. The pressure drop created by the expansion device causes a portion of the liquid refrigerant to evaporate and the resulting mixture of liquid and vapor refrigerant enters the liquid/vapor separator. Here the vapor is separated from the mixture and routed directly to the suction side of the compressor and the remaining liquid refrigerant enters the evaporator.
In the evaporator, the liquid refrigerant boils as it absorbs heat from water. The resulting vapor is drawn back to the compressor to repeat the cycle.

Pressure–Enthalpy (p-h) Chart
 
Pressure–Enthalpy (p-h) Chart:
The pressure-enthalpy chart is simply a plot of the saturated properties of a refrigerant. It plots refrigerant pressure (vertical axis) versus enthalpy (horizontal axis). Enthalpy is a measurement of the heat content, both sensible and latent, per pound [kg] of refrigerant.
For example, A represents the heat content of saturated liquid HFC-134a refrigerant at 147.5 psia [1.02 MPa] and 104°F [40°C]. B represents the heat content of saturated vapor HFC-134a refrigerant at the same pressure and temperature. The difference in heat content, or enthalpy, between A and B— that is, 70.2 Btu/pound [163.2 kJ/kg]—is the amount of heat required to
transform 1 pound [1 kg] of saturated liquid refrigerant to saturated refrigerant vapor at the same pressure and temperature.
If the heat content of the refrigerant at any pressure falls to the right of the curve, the vapor is superheated. Similarly, if the heat content of the refrigerant 
Refrigeration Cycle
Refrigeration Cycle:
 The theoretical vapor-compression refrigeration cycle for a helical-rotary water chiller can be plotted on a pressure-enthalpy chart.
The refrigerant leaves the evaporator as saturated vapor ➀ and flows to the suction end of the compressor where it enters the compartment for the suction- gas-cooled motor. Here the refrigerant flows across and cools the motor, then
enters the compression chamber. The refrigerant vapor is compressed in the compressor to a high pressure and temperature ➁. Energy input to the motor and compressor is imparted to the refrigerant as superheat. Superheated
refrigerant vapor leaves the compressor and enters the condenser.
Water or air flowing through the condenser absorbs heat from the hot, high- pressure refrigerant. This reduction in the heat content of the refrigerant vapor causes it to desuperheat ➂, condense into liquid ➃, and further sub-cool ➄ before leaving the condenser to travel to the expansion device.
The pressure drop created by the expansion process causes a portion of the liquid refrigerant to evaporate. The evaporating refrigerant absorbs heat from the remaining liquid refrigerant. The resulting mixture of cold liquid and vapor
refrigerant enters the liquid/vapor separator ➅. Here the vapor is separated
from the mixture and routed directly to the suction side of the compressor ➀
and the remaining liquid refrigerant enters the evaporator ➆.
The cool low-pressure liquid refrigerant enters the distribution system in the evaporator shell and is distributed over the tubes in the evaporator tube bundle, absorbing heat from water that flows through the tubes. This transfer
of heat boils the film of liquid refrigerant on the tube surfaces and the resulting vapor is drawn back to the compressor ➀ to repeat the cycle.

Refrigerants
Refrigerants:
Manufacturers are continuously improving their designs of helical-rotary water chillers. New chillers need to be designed around the characteristics of the refrigerant. Today there are 5 strong candidates for use with positive- displacement, helical-rotary chillers. They are HCFC-22, HFC-134a, HFC-404a, HFC-407c and HFC-410a.
 
refrigerants Thermodynamic Characteristics
 
 
refrigerants Thermodynamic Characteristics: 
Today the most commonly used refrigerant in helical-rotary chillers is HCFC-22. Due to the scheduled phaseout of HCFC-22, most helical-rotary chillers will be redesigned using HFC refrigerants. Many challenges are encountered when redesigning a chiller to use a refrigerant with different thermodynamic characteristics. This is due to the different efficiency, capacity, and operating pressure characteristics of each of the refrigerants.
Take a closer look at each of these issues by examining the effects of using different refrigerants in the same helical-rotary chiller designed for use with HCFC-22.
■    Efficiency – In order to meet today’s high standards of energy efficiency, chillers using refrigerants with a lower thermal efficiency, such as HFC-404a and HFC-410a, will require larger heat exchangers and more efficient compressors. These changes add to the product cost and increase the physical size of the chiller.
■    Capacity – HFC-134a has a lower capacity compared to HCFC-22, which means more refrigerant needs to be pumped through the chiller to achieve the same capacity. This can be accomplished by using a larger compressor or increasing the speed of the compressor. Both tend to increase product cost and design complexity. On the other hand, HFC-410a has a higher capacity compared to HCFC-22, which means less refrigerant needs to be pumped through the chiller to achieve the same capacity. The advantage is that smaller, less expensive compressors can be used.
■    Operating Pressure – A refrigerant that operates at a higher pressure requires heat exchangers and pressure vessels to be designed for the higher pressure. This adds cost. Conversely, higher pressure refrigerants have a greater density. As density increases, the required amount of refrigerant decreases, meaning that smaller or slower-speed compressors can be used. Lower pressure refrigerants, such as HFC-134a, will require larger or higher- speed compressors in order to achieve the same capacity as a similar chiller using HCFC-22.
                                                                abo bahaaeddine

3 comments:

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  2. merci pour cette information ami

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