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Keep Cool With Refrigerant Air Dryers

VSD Refrigerant Air Dryer

Across the country, the most common dryer present in compressed air systems is far and away the refrigerant air dryer. Throughout manufacturing and service industries, these machines are renowned workhorses. They are incredibly consistent, achieving dewpoints of ~38°F, which is more than enough to be used with pneumatic tools and other applications requiring moisture-free air. Air that has been dried by a refrigerant air dryer reaches ISO Quality classes 4 through 6. Please note that if you have not already been made aware of your ISO standards you shouldn’t have anything to worry about, but be sure to make sure you know before you purchase equipment. 

So this equipment is consistent and everywhere, but how exactly does this equipment function, and why do you need this equipment? For starters, when you compare this equipment with its main counterpart, the desiccant, refrigerant air dryers have:

  • Lower capital investment;
  • Lower operating and maintenance costs;
  • Higher dew points (more moisture left in the air)

Surprisingly enough, refrigerated air dryers use a refrigeration system to cool the air that is passing through. As this air cools, condensate is formed, and the air gets down to ~38°F, which explains the dewpoint. This process cools the air down to such a low temperature that moisture cannot be formed unless the air is below that temperature. So as long as your facility stays above 40°F, your compressed air will be dry and able to use without worry. After this condensate has formed, it is successfully drained out through a line in the bottom.

Now that you have a basic idea of how these compressed air dryers operate let’s dive into the nitty gritty details about these machines.

How Do These Refrigerant Air Dryers Work?

Let’s start with the basics, how does water get into your compressed air system? It isn’t raining outside, but somehow all of this moisture is being created in your equipment. So as air exists around us, it exists at atmospheric pressure, about 14.6 PSI at sea level. So if you have 1 square foot of air at atmospheric pressure, and then you increase that pressure to 150 PSI, you are reducing the space that air exists into a tenth of its size. So all the water molecules floating around in that space have been forced into a much smaller area.

As those molecules move into that smaller space, those like particles attract. So all of the water that was spaced out is combined with the air, but it is too hot after compression for the water vapor to condense. Once the air cools down, whether in the pipes, storage tank, or dryer, the water vapor condenses into liquid. Water is heavier than the air, so it falls to the bottom and must be removed. If your dryer is working properly there should be a condensate removal process in place to be filter and properly dispose of the water.

Now that we have the concept set, let’s follow the airflow through a refrigerant air dryer.

  1. Incoming air enters an air-to-air heat exchanger where the air that enters is cooled off by outgoing air that has been cooled already.
  2. Next the air goes into an air-to-refrigerant heat exchanger that drops the temperature of the air down to 38ºF from the liquid refrigerant.
  3. The cooling of the air causes the moisture to collect and condense. This water is then collected and is drained away.
  4. The air then finishes its path through the dryer by entering the air-to-air heat exchanger to cool down the incoming air and prevent the pipes from sweating.

Below is an illustration of this process working effectively and what happens when your system is not working as it should.

Key Characteristics To Be Aware Of

Maximum Pressure

-Your dryer’s maximum pressure needs to be equal or higher than your compressor’s maximum pressure.

Inlet Temperature

– Each dryer has its own specified maximum inlet temperature, and surpassing this can damage your equipment. If your compressor has an aftercooler, it will make sure the compressed air is at a reasonable temperature.

Maximum Flow

-This refers to the max amount of air that can pass through your dryer. If your dryer is undersized this can cause significant pressure drops due to the air being unable to pass through.

Maximum Room Temperature

-Keep your environment in mind with your equipment. If it’s hot outside, you run the risk of overheating your dryer. Just make sure your ambient air temperature is more than 20ºF below your max inlet temperature.

Refrigerant air dryers are the ideal solution for situations that:

  • Ambient temperature is less than 50°C/122°F,
  • Goal is to prevent condensation,
  • Required pressure dew point is around 3°C-4°C/37°F-39°F.

Calculating Your Pressure Dewpoint Temperature

  1. Determine your lowest ambient temperature
    1. Make sure to check areas that may be unheated areas underground and between buildings.
  2. Lower this temperature by 20°F
  3. This will give you your PDP that needs to be reached by your dryer
    1. If your lowest temperature is 65°F the your PDP would be 45°F

What Are The Different Types of Refrigerant Air Dryers

When it comes to selecting your refrigerant air dryer, there are a couple options that are available. With these options come differences, but not in how well they dry the air, but in how the equipment maintains its own internal temperature. There are three main types of refrigerant air dryer:

  • Non-Cycling
  • Cycling 
  • Variable Speed Drive


Non-Cycling Refrigerated Air Dryers

The littlest amount of technology, so the cheapest version of a refrigerant air dryer that you are going to be able to find is the non-cycling refrigerant air dryer. The non-cycling dryers’ refrigeration stays on continuously while the compressor is running. So if the compressor is on, the dryer will be running. If the dryer runs for too long without a break, it can potentially lead to freezing in the pipes and rapid wear on your machine.

However, if you have a smaller operation that does not need air all the time, these machines will work for you. If you just need clean dry air when you need it and don’t have a very large demand, overworking your machine won’t be the issue.

Cycling Refrigerated Air Dryers

The next step up is the cycling refrigerant air dryer. Cycling dryers cost more than their previous counterparts. That is because cycling dryers are designed to cycle the refrigeration on and off as needed to conserve energy and prevent excessive refrigeration when not being used. By cycling, it can prevent frost build up if the unit needs to run for a long period of time. 

These machines are great for the middle ground applications that may be running for a long time but do not have extremely high demand or operate for longer durations of time with typical flow. The technology in these dryers will be able to keep up with your demand as long as it is sized properly.

VSD Refrigerated Air Dryers

Variable Speed is the best of the best when it comes to compressed air technology. By being able to change operation speed to match demand, very little energy is wasted and you get the most out of your energy. A VSD refrigerant air dryer has a scroll compressor with a fixed element and an orbiting second element.

This compact screw quietly spins so that it can perfectly match the demand. This style of refrigeration is used to combat the fact that most compressed air systems do not operate under static flow and temperature conditions. This variability in climate is matched by the variable speed refrigeration. The ability to match dryer energy consumption to air demand helps provide significant savings in operating costs. 

The price of saving on operating costs is a higher upfront payment for the necessary technology and machinery to achieve this energy saving. If you can afford to go with the price increase it will be worth every penny when you save on your energy bill for months to come. As with all the equipment, the more you pay the better the results you will get, but that doesn’t mean the cheaper option cannot do the job.

Before You Buy

When you are considering your options for drying your compressed air, there are pros and cons for every option. Your final choice should be what works best for your system and nothing else. Going with a refrigerant air dryer is going to cost you less, and if your main priority is condensate prevention then it is perfect for the job. Issues arise when dew points below 37°F are needed or the ambient air gets above 122°F. 

These conditions can cause your refrigerant air dryer to fail, by potentially freezing if the temperature gets too low or being unable to keep up with the present demand due to excess heat and moisture. If the relative humidity of the air surpasses 50%, then your dryer will no longer be properly rated for the environment and let moisture through. This moisture will damage your system and your equipment. 

If you need Class 3 Air or Higher, please look into our desiccant air dryers, because refrigerant air dryers do not meet the quality standards necessary for that level of air quality. If you have extreme temperatures, excess humidity, and low dew point requirements are all reasons you would want to consider a desiccant air dryer. Read more about why you might need help Keeping The Water Out with desiccant air dryers.

Air Dryers are critical components of your process and you need to make sure you have the right type of dryer in place for your system. Protect your system and your wallet by keeping your air at the best quality and scheduling proper maintenance. Scheduled maintenance will always cost less than unforeseen downtime and issues.

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