The Evaporator
The purpose of the evaporator is to remove unwanted heat from the product, via the liquid refrigerant. The liquid
refrigerant contained within the evaporator is boiling at a low-pressure. The level of this pressure is determined by two
factors:
- The rate at which the heat is absorbed from the product to the liquid refrigerant in the evaporator
- The rate at which the low-pressure vapour is removed from the evaporator by the compressor
To enable the transfer of heat, the temperature of the liquid refrigerant must be lower than the temperature of the product
being cooled. Once transferred, the liquid refrigerant is drawn from the evaporator by the compressor via the suction line.
When leaving the evaporator coil the liquid refrigerant is in vapour form.
The Compressor
The purpose of the compressor is to draw the low-temperature, low-pressure vapour from the evaporator via the suction
line. Once drawn, the vapour is compressed. When vapour is compressed it rises in temperature. Therefore, the
compressor transforms the vapour from a low-temperature vapour to a high-temperature vapour, in turn increasing the
pressure. The vapour is then released from the compressor in to the discharge line.
The Condenser
The purpose of the condenser is to extract heat from the refrigerant to the outside air. The condenser is usually installed
on the reinforced roof of the building, which enables the transfer of heat. Fans mounted above the condenser unit are
used to draw air through the condenser coils.
The temperature of the high-pressure vapour determines the temperature at which the condensation begins. As heat has
to flow from the condenser to the air, the condensation temperature must be higher than that of the air; usually between -
12°C and -1°C. The high-pressure vapour within the condenser is then cooled to the point where it becomes a liquid
refrigerant once more, whilst retaining some heat. The liquid refrigerant then flows from the condenser in to the liquid line.
Source: http://www.europe.honeywell.com/70_refrigeration_control/EN5B-0024UK07%20R0505.pdf
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mercredi 2 juin 2010
mardi 1 juin 2010
Refrigerants in subcritical applications
In subcritical applications, refrigerant is metered by a capillary tube or thermostatic expansion valve, the control strategy being to inject liquid into the evaporator and maintain a given superheat entering the compressor. The metering device is selected or designed to ensure that there is complete evaporation ahead of the compressor.
Superheat is maintained to ensure that evaporator efficiency is optimal, and that liquid refrigerant does not enter the compressor. Excessive superheat may lead to overheating of the compressor.
In systems where a thermostatic expansion valve is used rather than a capillary tube, superheat is maintained by placement of a sensing bulb at the outlet of the evaporator. The modulation of the valve is then controlled by the temperature transmitted to it from the bulb and the pressure at an internal or external equalization port.
A different control strategy is needed in transcritical cycles.
A system based on the transcritical CO2 cycle uses a high pressure expansion valve (HPEV). Rather than controlling refrigerant metering from the low-pressure side of the system, modulation control comes from the high side of the system. A mechanical HPEV will control refrigerant injection into the evaporator by opening and closing based on the increase or decrease in gas cooler pressure.
In the HPEV, spring force is a closing force that acts on the top of a diaphragm. Increasing spring force throttles the valve, causing a backpressure in the gas cooler; the valve will not open until that back pressure, opposing spring force, increases to the point where it can overcome spring force and open the valve. The valve set point for the inlet pressure can be adjusted manually by compressing a spring in the valve.
Unlike a TEV, an HPEV does not control evaporator superheat. The HPEV injects refrigerant into the evaporator, but superheat is not directly controlled — instead it is indirectly regulated by system design.
The system charge, its distribution between the components, evaporator design, and the heat load, along with other external operating conditions, determines system superheat. By controlling the gas cooler pressure, the HPEV will indirectly influence system superheat, but the system must be designed so that liquid refrigerant in the evaporator outlet is not allowed to return to the compressor.
The HPEV was designed to control gas cooler pressure rather than suction line superheat as does a TEV. An HPEV, therefore, must withstand high-side CO2 pressures that can reach 1500 psia, at the same time accurately controlling gas cooler pressure. Slight capacity and energy efficiency (COP).
Superheat is maintained to ensure that evaporator efficiency is optimal, and that liquid refrigerant does not enter the compressor. Excessive superheat may lead to overheating of the compressor.
In systems where a thermostatic expansion valve is used rather than a capillary tube, superheat is maintained by placement of a sensing bulb at the outlet of the evaporator. The modulation of the valve is then controlled by the temperature transmitted to it from the bulb and the pressure at an internal or external equalization port.
A different control strategy is needed in transcritical cycles.
A system based on the transcritical CO2 cycle uses a high pressure expansion valve (HPEV). Rather than controlling refrigerant metering from the low-pressure side of the system, modulation control comes from the high side of the system. A mechanical HPEV will control refrigerant injection into the evaporator by opening and closing based on the increase or decrease in gas cooler pressure.
In the HPEV, spring force is a closing force that acts on the top of a diaphragm. Increasing spring force throttles the valve, causing a backpressure in the gas cooler; the valve will not open until that back pressure, opposing spring force, increases to the point where it can overcome spring force and open the valve. The valve set point for the inlet pressure can be adjusted manually by compressing a spring in the valve.
Unlike a TEV, an HPEV does not control evaporator superheat. The HPEV injects refrigerant into the evaporator, but superheat is not directly controlled — instead it is indirectly regulated by system design.
The system charge, its distribution between the components, evaporator design, and the heat load, along with other external operating conditions, determines system superheat. By controlling the gas cooler pressure, the HPEV will indirectly influence system superheat, but the system must be designed so that liquid refrigerant in the evaporator outlet is not allowed to return to the compressor.
The HPEV was designed to control gas cooler pressure rather than suction line superheat as does a TEV. An HPEV, therefore, must withstand high-side CO2 pressures that can reach 1500 psia, at the same time accurately controlling gas cooler pressure. Slight capacity and energy efficiency (COP).
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