Refrigerated Air Dryer

April 27, 2016Ken Bean

Last week we covered Compressed Air Dryer – 101 which gave an overview of why air dryers are needed and how water enters the compressed air system.

Today we review the most common category of compressed air dryer, the refrigerated air dryer.

While the refrigerated dryer is by far the most widely used category of compressed air dryer, it does not offer as low a dew point as can be attained with other categories. The big selling feature for the refrigerated dryer is the the low initial investment in conjunction with a usually acceptable dew point for a wide range of industrial applications.

The principle of operation is much like a typical home refrigerator or residential air conditioner system. The compressed air is cooled in an air-to-refrigerant heat exchanger down to about 35⁰F. The moisture is condensed to a liquid state where it is removed by an internal separator and then drained from the unit with an automatic drain. The compressed air is then reheated in an air-to-air heat exchanger where the incoming compressed air first passes. This also pre-cools the incoming compressed air thus reducing the load on the refrigeration system.

This flow path allows the compressed air to exit the dryer with a pressure dew point of 35⁰ to 40⁰F. A lower dew point is not feasible with a refrigerated dryer as the condensate would freeze at 32⁰F or lower.

The advantages of refrigerant type dryers include:

Low, initial capital investment
Low operating cost
Low maintenance cost
No damage caused by oil in the air stream (*filtration is still typically recommended)

Disadvantages of refrigerant type dryers include:

Limited dew point suppression

There are a few different types of dryers in the refrigerated classification.

The first being the non-cycling type. In a non-cycling type refrigerated dryer, the refrigerant circulates continuously through the system. This design provides quick response to changes in operating loads (flow rates, ambient temperatures & pressures). Since the flow of compressed air will continuously vary and ambient temperatures also vary, a hot gas bypass valve or unloader valve is often used to regulate the flow of the refrigerant and maintain stable operating conditions within the refrigerant system. In most designs, the refrigerant evaporates within the air-to-refrigerant heat exchanger and is condensed after compression by an air or water-to-refrigerant heat exchanger.

The schematic below offers an overview of the system flows.

While older types of refrigerated dryers have utilized CFC refrigerants such as R12 and/or R22, todays newer designs have moved to incorporate chlorine free refrigerants such as R134, R407 or other environmentally friendly refrigerants making this a non-issue for todays industrial plants. However, the use of the new refrigerant types do require careful attention to the refrigerant system designs due to differences in operating pressures and temperatures. These and all refrigerated type air dryers should be serviced only by a licensed and trained technician to assure compliance with refrigerant recovery and release regulations.

Advantages of non-cycling refrigerated dryers include:

Minimal dew point fluctuation
Refrigerant compressor operates continuously

Disadvantages of non-cycling refrigerated dryers include:

No energy savings at part load and/or no flow conditions

The next type of refrigerated air dryer is termed a “cycling type dryer” which use the refrigerant to chill a mass of material surrounding the compressed air passages in the heat exchanger. The mass may be a liquid such as glycol or metal such as aluminum block, beads or other type of material, which act as a heat sink. The compressed air is cooled by the heat sink which has its temperature controlled by a thermostat and shuts off the refrigerant compressor during reduced loads, providing savings in operating cost. The detriment to the cycling type dryer is the higher initial investment.

Advantages of cycling type refrigerated dryers include:

Energy savings at part load and/or no flow condition

Disadvantages of cycling type refrigerated dryers include:

Dew Point fluctuation
Increased size and weight to accommodate the heat sink mass
Higher initial investment

The final type of refrigerated dryer I wanted to discuss is relatively new on the scene and what I call a load matching dryer. This dryer utilizes a digital scroll type type compressor which is capable of unloading. Thereby offering reduced energy consumption by matching the refrigeration requirement to the flow rate of the moment in conjunction with the ambient temperature. A tremendous feature of these units is they are modular so the modules can be connected to create the size of system required. The modules for this particular type of dryer come in 2 sizes, 1250 SCFM and 2500 SCFM. Each module can be completely isolated for maintenance. Systems can be built from 1250 SCFM up to 12,500 SCFM. It is important to note that standard cycling type or thermal mass refrigerated dryers are also available in modular forms but do not offer the digital scroll (unloading type) refrigeration compressors.

Advantages of load matching type refrigerated dryers include:

Pressure dew point stability from 0% to 100% load
Instantaneous response to load changes
No dew point spikes

Disadvantages of load matching type refrigerated dryers include:

Small sizes are not available
Higher initial investment (comparable to non-cycling type)

Compressed Air Dryer – 101

April 19, 2016April 19, 2016Ken Bean

Most all industrial compressed air systems require some type of dryer. The reason is simple, to remove water from the compressed air prior to use downstream.

I saw a recent advertisement with the caption:

I’m not sure how one can determine the “best ways” to dry compressed air without a complete evaluation of the air usage

First things first:

Let’s look at where the water comes from?

Water comes from the atmospheric air that the compressor ingest through the intake. Keep in mind that most plants will look to have compressed air pressure at 100 PSIG. To achieve this pressure the compressor runs a compression ratio of 8 to 1 depending on your physical location. Meaning it must ingest 8 cubic feet of atmospheric air and compress this into 1 cubic foot to achieve the desire pressure.

When you compress 8 cubic feet of air your compressor also compresses all of the contaminants that were in that 8 cubic feet into one cubic foot, including the water that naturally exists in the atmosphere. Its important to know that the air’s ability to hold moisture in a vapor state is directly related to it’s temperature and pressure. In the atmosphere where the pressure is nominally 14.5 PSIA, water will stay in a vapor state until the air reaches it’s saturation point. ie. all the water it can hold at a given temperature (our pressure is already set at 14.5 PSIA). Once this level is exceeded – it rains. Or the water that was previously in a vapor state now converts to liquid form.

The same circumstance is happening inside the pipes of your compressed air system. Once you compress all of the contaminant (in this case water) into a single cubic foot, even with the increase in temperature and pressure, there is still more water than the air can hold in a vapor state so it rain inside your pipes. Additionally as the temperature drops the air’s moisture holding capacity will be lowered and water will continue to condense out as a liquid.

Water is extremely detrimental to compressed air uses downstream. To determine what type of dryer your system will require mandates a look at the possible end uses downstream. For example, if the air use downstream is solely to blow metal chips from a machining operation from a blow gun then chances are the water condensing out will not cause a problem. If however, you happen to have robotic paint spraying systems downstream then any water will be devastating not only to the robotics but will likely also ruin the paint finish when water mixes with the coating during the spraying process.

An additional consideration should also be reviewed, the ambient conditions! In your area are freezing temperatures observed at any time during the year? If the answer is yes then are any compressed air lines exposed to outdoor temperatures. For example, if the above mentioned application of blowing metal chips from a machining operation is the compressed air use (which wouldn’t require a dryer), but the compressors are located in a remote building and the pipe travels outside to the machining building then it would be likely that the air line between buildings could freeze in the winter thus shutting down your machining operation.

Once an assessment has been performed to review the uses of compressed air within the plant along with any ambient temperature issue’s, only then can a determination be made as to the type of dryer required, if any.

Obviously there are thousands of potential uses for compressed air and each one comes with it’s own particular concerns. Great news for end users is where air is being used for equipment, the equipment manufacturer will normally provide direction on the quality of air required for their particular machines.

One other consideration to keep in mind while looking to determine the air dryer requirements is the contaminants in the compressed air stream are not limited to water only. Thinking back to our requirement of 8 cubic feet of atmospheric air being ingested into the compressor is that all contaminants in the atmosphere are magnified 8 times during the compression cycle. This mean any particulate, hydrocarbons or other gas fumes that are in proximity of the compressor intake will also be ingested into the compressor. While our series of posts related to air dryers will not cross over to other contaminants it is worth noting that filtration selection usually goes hand in hand with selecting the air dryer.

In the world of compressed air dryers there are 4 basic classifications of dryers that are normally associated with industrial compressed air systems.

Refrigerated Type Dryers

Refrigerated dryers: The principle of operation is similar to a domestic refrigerator or home air conditioning system. The compressed air is cooled in an air-to-refrigerant heat exchanger to about 35°F, at which point the condensed moisture is separated and drained off.

Regenerative Type Desiccant Dryers (adsorbing)

These dryers use a desiccant, which adsorbs the water vapor in the air stream.

Deliquescent Type Air Dryers (absorbing)

The deliquescent desiccant type dryer uses a hygroscopic desiccant material having a high affinity for water. The desiccant absorbs the water vapor and is dissolved in the liquid formed.

Membrane Type Air Dryers

Specially designed membranes allow water vapor (a gas) to pass through the membrane pores faster than the other gases (air) reducing the amount of water vapor in the air stream at the outlet of the membrane dryer, suppressing the dew point.

Within each classification of air dryer there are different types, each with it’s own operating characteristics. While each type will provide the same end result, their operational modes are different as to be matched to each plants needs and energy requirements as well as initial investment costs.

With the goal of our blog posts to offer small bite size pieces of information that our users can easily digest, we will be writing separate posts for each classification of dryer over the next few weeks. This will allow our readers to learn about each classification and type of dryer as their needs require or the ability to review all of the posts for a comparative overview.

As always, if there are questions, feel free to contact us for additional assistance.

Do You Have a Matched System

April 12, 2016Ken Bean

Simply put, does the drawing left of the red line (Supply) match the right side (Demand) in terms of air flow and pressure? Does it continually match? Likely it does not as demand is in constant fluctuation whether from changes in use, shift change or downtime.

Within a compressed air system, the system dynamics (changes in demand over time) are especially important. Using controls, storage, and demand management to effectively design a system that meets peak requirements but also operates efficiently at part-load is key to a high performance compressed air system.

In many systems, compressor controls are not coordinated to meet the demand requirements, which can result in compressors operating in conflict with each other, short-cycling, or blowing off are all signs of inefficient system operation.

As in most relationships, the key is communication. Communication between the compressors so that each (or a central controller) knows the status of all the other units. Communication is also key between the compressors (or central controller) and the demand side of the system. The compressors perform based on information from a pressure sensor located downstream of the compressor discharge. But where is this sensor located?

A common mistake is the pressure sensor is located close to the compressor discharge. Perhaps even before the dryer. If this is the case in our system drawing above, you can see that the compressor is performing based on pressure data before the air enters the dryer or the subsequent filter, both of which have pressure drop as air passes through. This drop can range from 5 PSIG up to 12 PSIG or higher depending on how well the system was designed as well as the condition of the filter element. A dirty element near the end of it’s life will have substantially more pressure differential than a clean filter element.

If the compressors are discharging at 100 PSIG and there is a 10 pound drop through the dryer and filter, then the plant is only receiving 90 PSIG and this doesn’t include the pressure drop throughout the plant piping system. When a cyclical event occurs, such as several blow guns operating at the same time, the pressure drop has to travel back through the entire system before the compressors see the event and can respond to correct the drop in pressure by increasing their output.

All factors must be considered when designing or updating a compressed air system. Your best course of action is to consult with a compressed air professional to assist in your design or upgrades prior to writing any equipment specifications.

Compressor 1, Compressor 2, Compressor 3…You Get The Idea

April 5, 2016Ken Bean

With a room full of compressors, how do you keep them all playing together nicely? Central controls is the key to an energy efficient system with multiple compressor. Even with various types and brands a central control system can form a chain of command that keeps the plant satisfied while reducing energy costs.

The simplest central control is utilizing the existing compressor controllers for communication between the compressors allowing one or two units to focus as the lead machine and calling on the remaining compressors to assist in high load conditions. This could be by automatically loading or modulating one or several of the remaining units.

System master controls have many functional capabilities, including the ability to monitor and control all components in the system as well as trending data to enhance maintenance functions and minimize costs of operation.

What if your compressors are from multiple manufacturers and the controllers will not communicate with each other? Time to call on an outside source such as IZ Systems. The IZ Systems central control system can communicate with all compressor manufacturers controls and allow many enhanced functions not usually associated with normal compressor controls such as web based control, real time energy consumption and even contracts allowing our specialist to monitor your compressed air components for you remotely, giving you piece of mind 24/7/365.