Key Takeaways
- False Load Concept: Hot gas bypass valves add artificial heat load to evaporators to maintain optimal operating conditions when actual cooling demand is low
- System Protection: Prevents compressor short cycling and evaporator freezing during low-load conditions, extending equipment life
- Two Types Available: Mechanical valves offer simple manual adjustment while electronic valves provide precise control through stepper motors and controllers
- Critical Piping: Three main configurations exist – evaporator inlet with distributor, without distributor, or direct suction line injection – each with specific considerations for oil return
Hot gas bypass valves are a simple and very effective way to add a false load on an evaporator coil. What is a false load, you ask? As we have learned in the past, refrigerant pressures are directly related to the ambient temperature surrounding them.
For example, if the return air in a system is close to the set point, regardless of what that set point is, the suction pressure will be lower than if the return air was five degrees above the set point. By adding hot gas from the discharge line into the evaporator, we can increase the evaporator pressure and temperature. Because heat from the return air does not put this load there, the load is referred to as false.
Understanding Variable Load Applications
Hot gas bypass valves can be used in many different applications where the load on the evaporator varies, from residential to server rooms to process cooling equipment. A hot gas bypass valve can also prevent short cycling of a compressor and evaporator freezing in low load conditions.
As a space or process temperature reaches a set point, hot gas can be added to the evaporator to raise its temperature. With increased evaporator temperature, the set point will take longer to achieve, thus increasing compressor run times while preventing evaporator freeze-up. However, it’s normal in some applications for evaporators to freeze during normal operation, like in low-temperature refrigeration applications where proper system evacuation becomes even more critical.
Real-World Application Example
A large lecture hall is filled with people, the load will be high, and the system will most likely run at 100% capacity to cool the space. During an intermission, people leave for a break, the load drops, and the room approaches a set point.
The hot gas bypass valve opens, putting a false load on the evaporator. Since the evaporator temperature has been increased, the supply air serving the lecture hall will also be increased, preventing the set point from being reached.
As people re-enter the hall, a true load will increase evaporator temperature, and the hot gas bypass will close. This prevents compressor short cycling with varying loads; compressor stops and starts are a major factor in compressor failure over time.
Let’s take a look at how the valve operates. There are mechanical valves and electrical valves.
Whiteboard Explanation of a Hot Gas Bypass Valve
Mechanical Hot Gas Bypass Valves
The mechanical hot gas bypass valve (HGB) is fed discharge gas (hot gas) teed off from the discharge line. The output of the valve is directly piped to the inlet of the evaporator after the TX valve. This differs from bi-flow TXVs in heat pumps which handle refrigerant flow in both directions.
In some situations where multiple evaporators are used, it may be piped into the suction line. However, this has some disadvantages, like improper oil return to the compressor. They are adjusted manually; the Sporlan ADRSE-2, from the video below, is adjusted using an Allen key.
Adjusting the valve inwards will allow more hot gas to flow through it, and backing out the adjustment will allow less hot gas to flow through it. You may need to simulate a low-load condition when setting up the valve.
For instance, if you want to maintain a suction pressure of 60 psi in the evaporator, perform the following steps:
- Simulate a low load condition, allowing the pressure to drop below 60 psi
- Starting with the hot gas adjustment backed all the way out, slowly adjust the valve inwards
- Continue adjusting until the flow of hot gas raises the evaporator pressure to the desired setting of 60 psi
- The valve will maintain that set point by monitoring evaporator pressure and adding hot gas as needed
Electronic Hot Gas Bypass Valves
The electronic hot gas bypass valve (EHGB) uses the same concept as its mechanical counterpart but operates with greater precision. For example, the Sporlan SDR series uses a 12 VDC stepper motor.
A controller must be used in conjunction with the valve and be capable of monitoring temperature and pressure. The electronic setup can be used for more accurate control of evaporator temperature, similar to how BMS network architecture provides precise control in commercial systems. The electronic valve can also be piped to the evaporator inlet or the suction line.
You will need an interface to set up this valve; an Allen key will not suffice.
Piping Configurations
Generally speaking, there are three ways of piping in a hot gas bypass line:
1. Evaporator Inlet with a Distributor
With this method, the hot gas bypass line shall be piped after the TX valve but before the distributor. An auxiliary side connection (ASC) should be used. The ASC prevents TX valve operating issues as hot gas flows into the evaporator.
2. Evaporator Inlet with No Distributor
This configuration follows the same principles as above, maintaining proper refrigerant distribution while preventing TX valve interference.
3. Directly into the Suction Line
This configuration can be employed when multiple evaporators are used. It should be noted that oil return to the compressor could be an issue due to refrigerant bypassing the evaporator coil.
Not all valves are created equal; refer to manufacturer specifications, installation, and service literature before choosing or setting up a hot gas bypass valve. Understanding how refrigerant works in these various configurations is essential for proper installation.
Conclusion
Hot gas bypass valves play a crucial role in maintaining the efficiency and reliability of refrigeration and air conditioning systems. By adding a false load, these valves prevent evaporator coil freezing and compressor short cycling, especially under varying load conditions.
Understanding their operation, proper selection, and installation is essential for optimal system performance. Regular maintenance and adherence to manufacturer specifications ensure these valves function effectively, safeguarding your system’s longevity and efficiency.
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