Morning Sun HVAC design mini program
First, the role and practice of the hot gas bypass valve
The energy regulation of the refrigeration system is generally composed of the following methods
Compressor paralleling - multi-stage control;
Variable speed compressor - frequency conversion, etc.;
Compressors with unloading devices;
Hot air bypass.
For compressor refrigeration systems that do not have their own energy regulation devices, the use of hot gas bypass for system energy regulation is very economical, and the hot gas bypass valve provides a practical, economical and reliable energy regulation solution. [Source: Refrigeration Encyclopedia] is particularly suitable for compressors without their own energy regulation mechanism. The suction pressure is better maintained at low loads, preventing the evaporator from freezing and maintaining better oil return performance. General adjustment range: 0 0.55MPa; Safe operating pressure - 2.75MPa (400psig).
Two common practices for hot gas bypass valves:
1. Direct bypass to the return air end;
2. Bypass to the entrance of the evaporator;
Bypass directly to the return air end:
As shown in the figure below, the inlet of the bypass to the evaporator is connected to the suction pipe, without passing through the evaporator and directly through the suction pipe back to the compressor, the high temperature and high pressure of the hot gas is mixed with the low temperature and low pressure gas refrigerant coming out of the evaporator, which improves the suction pressure of the compressor.
However, this practice will have a negative impact, too high suction temperature will cause damage to the operation of the compressor (as shown in the figure below), may shorten the service life of the compressor, so what to do?
We can add a spray cooling solenoid valve, when the suction temperature is too high, you can control the solenoid valve to reduce the suction temperature of the compressor, as shown in the following figure:
Bypass to the return end
Of course, we can also take the second method, bypassing to the inlet of the evaporator.
Bypass to the inlet of the evaporator:
The position where the bypass solenoid is connected to the high-pressure side can be located anywhere on the line before entering the condenser. Connecting the low-pressure side pipeline, so that the high-temperature and high-pressure hot gas passing from the high-pressure side enters the evaporator, increasing the load of the evaporator, which can prevent the evaporator from being frozen due to the evaporation pressure falling, [Source: Refrigeration Encyclopedia] Also improves the outlet temperature and pressure of the evaporator, and improves the operating conditions of the compressor under low temperature operation.
Bypass directly to the evaporator inlet
Second, the regulation of the hot gas bypass valve
The first step after the installation of the hot air bypass valve is to determine the minimum suction pressure under the operating conditions as the setting point of the hot gas bypass valve, and the adjustment steps:
1. Install a pressure gauge on the compressor suction pipe and measure the suction pressure after the system is stabilized. To determine that there is no bypass of the hot gas at this time, listen to whether there is a sound of air flowing through the valve or touch the outlet line of the valve, if the temperature of the outlet line is high, it means that there is a hot gas bypass.
2. Prevent hot gas bypass. The power to the hot gas bypass line solenoid valve can be cut off or the control lever can be adjusted to keep the hot gas bypass valve in a completely closed state.
3. Reduce the load of the evaporator until the suction pressure is reduced to the set point where hot gas bypass is required. The load on the evaporator can be reduced by closing the air outlet, covering the evaporator, reducing the flow of air, shutting down the fan, and turning off the bypass water of the water cooler.
4. Ensure that the solenoid valve of the hot gas bypass pipeline at this time is in the open state. Adjust the adjustment lever of the hot gas bypass valve so that the hot gas bypass valve begins to bypass the hot air until the suction pressure no longer drops and remains at the expected value.
Regulation mode of hot gas bypass valve: clockwise adjustment of the adjustment lever will increase the pressure set value; Adjusting the adjustment lever counterclockwise will reduce the pressure setpoint; Adjustments should be made slowly and slightly to allow the system to run stably after each turn of adjustment.
3. Precautions
For parallel evaporator systems, the hot gas bypass shall not be greater than the cooling capacity of a single evaporator, and shall be controlled by a multi-channel hot gas bypass valve. [Source: Refrigeration Encyclopedia] If the compressor is located in the upper part of the evaporator, the hot gas bypass to the evaporator must ensure that the compressor has sufficient oil return capacity under low load bypass conditions.
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