Understanding How Undersized Dryer Components Can Reduce Air Quality


Van Air Systems was recently approached by a customer at the SSPC show to discuss the differences between compressed-air drying systems. He spoke at great length about the size of the air motor on our 1600 cfm (cubic feet per minute) air dryer package, saying he was surprised that, in a package he had purchased from another manufacturer, the air motor was half the size of the Van Air Systems unit.

After sending out his company's new dryer on a few rentals, he told us, maintenance had become a burden. The smaller air motor had to work harder than larger ones to spin at the same RPMs and achieve the same approach-temperature performance. As a result, there was additional wear and tear on the air motor, causing premature failure, and the customer was forced to make a tough decision — rebuild the air motor or spend the money to replace the current air motor with a larger, more suitable size. This is one of many examples that show why size matters in our business.

This guide was written to help you and your customers purchase a properly sized air dryer system that meets your application requirements.

One of the biggest issues we come across when helping customers is their units not working properly due to water in their airlines. This is usually caused by an incorrectly sized unit.

Since the dryer vessel size is the most critical part of any dryer package, it can also be the most expensive component. Often times, manufacturers will undersize a dryer vessel or use thinner steel to remove some of the cost of its portable dryer packages.

Knowing operating flow rates and pressures is essential to sizing a compressed-air dryer system properly. Before buying, consider the following, and save yourself from headaches due to problems with an undersized dryer package.

  1. What is the dewpoint you need to achieve? Or what problem are you trying to solve with your compressed-air dryer? Dry, compressed air is a concept with a wide range of meanings. Knowing your dewpoint or the application in which you will be using your dryer will help you make sure its sized properly. Dewpoint — the temperature at which moisture begins to drop out of the air — signals how dry the compressed air is. The goal is to always keep your dewpoint temperature lower than your compressed-air temperature. To do this, you need to make sure your dryer is drying the air properly. For example, dewpoint suppression is the primary purpose of all deliquescent dryers and deliquescent desiccants. If you maintain the desired dewpoint, you will never have a problem with water in your lines.
  2. What size compressor do you have? Believe it or not, it is common for owners to be unfamiliar with how an air dryer works. We have had some customers give us the height, width and depth of their compressor, rather than the capacity, when we ask for its size. If you are unsure of the capacity of a compressor, first determine the horsepower of the machine. You can get up to 5 scfm (standard cubic feet per minute) of compressed air for each horsepower. This makes sizing your dryer a lot easier.
  3. What is your operating pressure? This is a very important question. You will want to know both the low and high values for this. You can size your equipment for the lowest value, but it's important to be sure that the highest value is within the design range for the filter and dryer. One of the main reasons that dryers fail is they are sized improperly for the operating pressure under which they will run. Determine your flow rate at the pressure you intend to operate. Higher pressures will be able to flow larger scfm amounts. If your dryer is not capable of handling your flow at the required operating pressure, performance may suffer.
  • In a nutshell, the bigger the vessel, the more desiccant it can hold and the more scfm you can flow through the dryer without sacrificing performance. The longer the air remains in contact with the desiccant, the lower the dewpoint and the better your dewpoint performance will be at the outlet. Since a larger dryer vessel will hold more desiccant, you will also be able to extend maintenance cycles.
  • Below is a recommended vessel size based on flow at 100 psig*, 125 psig and 150 psig.
Dryer Example: Van Air Blast Pak Model # Vessel Diameter SCFM Flow Rate @ 100 psig SCFM Flow Rate @ 125 psig SCFM Flow Rate @ 150 psig
PRO-25 16"  250 304 359
PRO-40 20"  400 487 574
PRO-50 24"  800 974 1149
PRO-75 30"  1200 1462 1723
PRO-100 36"  1600 1949 2297

*per square inch gauge

  • Where will you find your operating pressure? Most production machinery lists air consumption in the specifications or operating manual. Digging through old files to find a manual may be tedious, but it's a sure way to avoid a sizing mistake for the filter and dryer. Plan B is to put a flow meter in the line and actually measure what is going through it. In most applications, the price to rent a flow meter is cheap insurance versus spending money for equipment that's too big or small and not getting the job done properly.
  1. What is your air temperature at the inlet? Temperature is the final critical area. Being off by just a 10- or 20-percent Fahrenheit error in either direction means that there is twice as much moisture in your system than you thought. Your brand-new dryer is now too small to keep up with the moisture load. The old-time method of touching the pipe with your hand is, at best, not that accurate. The quickest and most effective way to measure compressed-air temperature is with one of the infrared devices available at home supply stores for a reasonable price. A benefit of using an infrared device is that you do not need to get close to or actually touch that filthy compressor or piping for an accurate reading; you will have a digital result. 

You Get What You Pay For

Purchasing an air dryer system is an investment, so make sure it will outlast any application. Getting a properly sized unit may cost a little more up front, but it will save you from breakdowns in the future.

In the rental market, longevity of the equipment translates to higher profit from your purchase. Spending a little more on the initial purchase to get a high-quality dryer package with oversized components will eliminate down time, improve performance and reduce maintenance costs. Ultimately, it will be more profitable over the 15-year life of the equipment to base your decision on quality and not price alone.

The Van Air Systems technical team is available to share its experiences (good and bad) to help your customers get the appropriate size dryer for the application.

Content provided by Van Air Systems.

*Claims or positions expressed by sponsoring authors do not necessarily reflect the views of TPC, PaintSquare or its editors.