As cold weather approaches, we look for ways to stay warmer, whether that be wearing additional layers, vamping up the heating, or building fires in our fireplaces. Cold weather can cause problems with refrigerant dryers, and so compressed air users look for ways to avoid the trouble caused by freezing temperatures.

This article will provide you with all the relevant information on the benefits of using hot gas bypass on a refrigerant air dryer.

Table of Contents

What is Hot Gas Bypass?

Hot gas bypass, or HGBP for short, is essentially a valve that automatically regulates the cooling temperature in the refrigerant air dryer exchanger based on the pressure of the refrigerant. There is a direct correlation between the pressure measured in the heat exchanger and the temperature of the system, and so, if there is a low pressure there will be a low temperature, and vice versa.

Refrigerant driers typically have a lower system pressure when they’re partially loaded. The hot gas bypass valve opens when the pressure in the refrigerant circuit decreases to allow some of the refrigerant to circumvent the heat exchanger, which helps to eliminate the problem of over-cooling. Hot gas bypass valve adjustment allows you to keep your dryer running at its optimal temperature without risking the health of your system. For more information on refrigerant dryers, visit our Refrigerated Air Dryer Guide!

When diminishing loads force a refrigeration system to operate at unstable conditions, compressor and evaporator capacities balance at even lower suction pressures and temperatures. If unchecked, this will eventually result is coil frosting and compressor flooding.

Hot gas bypass valves can stabilize the system balance point by diverting hot, high-pressure refrigerant vapor from the discharge line directly to the low-pressure side of the system. This helps to keep the compressor more fully loaded while the evaporator satisfies the part-load condition of the dryer. Also, and very importantly, the diverted vapor raises the suction temperature, which prevents frost from forming in the dryer and protecting it.

What is the Problem with Over-cooling?

The refrigeration circuit found within refrigerant dryers is sized to cool the full inlet capacity to a certain specified temperature. In the case that there is only a partial load on the dryer, the refrigerant is likely to over-cool the air which can become a serious problem.

Over-cooled air will cause the water in the air to not just condense, but possibly even freeze. Now, water freezing within a heat exchanger cab block the airflow and cause expansion due to ice formation which will eventually lead to cracks forming on the heat exchanger. We have a guide on Running An Air Compressor In Cold Weather Months which may be of interest to you!

Hot Gas Bypass Selection Considerations

When selecting a hot gas bypass suitable for your refrigerant air dryer, it’s important to choose one that is capable of being fully open and closed based on the pressure input. Some people instantly go and purchase one without looking at the specifics, and in the case that you purchase a hot gas bypass valve that has limited opening capabilities, well, it’s less likely you will be able to prevent freezing when the incoming load is especially low.

The control of the opening of the hot gas bypass valve has to be a function of the measured pressure and the valve position needs to be consistent. For the dryer to maintain a steady output pressure dewpoint, this is an absolute requirement. A dryer should be fit for life, in that all the components are maintained and remain reliable over many years of operation, including the hot gas bypass itself.

For those unsure on the dew point, it’s the temperature at which water vapor in your compressed air changes into liquid form and becomes water droplets (condensation). For information on How to Measure Dew Point & What is Dew Point in Compressed Air Systems? visit our guide!

It’s important to select an evaporator coil that is able to maintain a high suction temperature to permit the system to aggressively stage down before frost conditions develop. Coils with intertwined circuits tend to reduce the risk of coil frosting because they use more of the available fin furnace at part-load conditions.

There are two basic types of HGBP valves, proportional and electronic. Proportional pressure-actuated HGBP valves open on a set amount of pressure difference between the valve’s set suction pressure and the actual suction pressure. Electronic HGBP valves can regulate the bypass flow and maintain the desired suction pressure or discharge air pressure or discharge air temperature.

One problem, of course, is that you can’t escape the cold weather that winter brings. But, leaving your refrigerant dryer in the cold is not an option. By ensuring that your refrigerant dryer has and utilizes a hot gas bypass, you can prevent any serious harm done by over-cooling. In most cases, they need around a 6 psig difference to open properly.

FAQs (Frequently Asked Questions)

What is hot gas bypass used for?

Hot gas bypass valves are used on refrigerant dryers to prevent the over-cooling of the system which can lead to frosting. A hot gas bypass valve essentially allows you to keep your dryer running at its optimal temperature without risking the health of your system.

Does gas bypass need to be hot?

It’s possible to get both cold gas bypass valves and hot gas bypass valves. The latter is the more commonly used and found on refrigerated air dryers. The former is more likely to be found on different applications like automated compressor surge recovery in gas turbine hybrid systems.

Is hot gas bypass the same as hot gas reheat?

These two terms are commonly misunderstood. Hot gas bypass provides an artificial load by introducing a portion of high pressure, high-temperature air to the suction side of the system, helping to keep the system from freezing. Hot gas reheat uses high pressure, high-temperature compressor discharge gas to reheat air leaving the dryers coil and dehumidify it to help reach the desired temperature setpoint, not to specifically stop the system from freezing as a hot gas bypass does.

Does hot gas bypass dehumidify?

No, it is hot gas reheat that is typically used in dehumidification modes to meet desired space sensible temperature points no the hot gas bypass.


If you have any questions regarding using hot gas bypass on refrigerant dryers, please leave a comment below, with a photo if applicable, so that someone can help you!