Double the Performance of Dehumidifiers

December 1, 2006
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Chart 1

The focus lately in water restoration has been on increasing evaporation in materials to speed drying. Increasing evaporation is obviously a key to drying structures faster. But now that we can increase the speed of evaporation, remember that all that water has to go somewhere. The job of dehumidifiers and open-air drying is to remove the water that is evaporated. But how much water is being removed? Is it enough? And most importantly for your bottom line, how can you get more performance out of your current equipment?

What Is Grain Depression?

The basic measure of performance on a dehumidifier is grain depression. Grain depression is the difference in the humidity ratio (GPP) of the air entering and the air leaving a dehumidifier. Generally speaking, the more grain depression, the better the dehumidifier is working.

Grain depression is a simple measurement to take. Compare the affected air GPP to the air coming out of the dehumidifier. Some common mistakes made when measuring grain depression include:
  • Using %RH for the reading –  %RH has little meaning when comparing different air streams.
  • Improper thermo-hygrometer technique – Allow the meter to fully acclimate.
  • Wrong position of the thermo-hygrometer – Insert the probe as far into the outlet of the dehumidifier as it will go. (If you hear a grinding sound, back it up!)

Be sure to determine the GPP onsite. Many companies document temperature and relative humidity, but don’t calculate GPP until returning to the shop. What if you find out that the dehumidifier isn’t working? It isn’t cost effective to make a second trip to the job that day, so the underperforming dehumidifier will wait until tomorrow. Not a good idea!

A better alternative is to properly train employees to monitor and document temperature, RH, and GPP. Also, train employees to make the next step and look for grain depression in every dehumidifier placed on a water loss.

Whenever using grain depression, keep in mind that it is a very basic measurement. In many ways it is a relative reading that can be misinterpreted. Grain depression must be evaluated hand in hand with the amount of air (CFM) that a dehumidifier moves in order to get a full picture of the true effect of dehumidifier performance. For example, these readings were from a test performed yesterday (Chart 1). Based on grain depression alone, the decision might be made to remove the Dehumidifier A because it is “performing less.” This would be the wrong choice.

Chart 2

Estimating Pint Removal: For the Math Wiz

To have a full understanding of how well a dehumidifier is working, grain depression must be combined with CFM. If you’re not a “drying Godzilla” just skip the next paragraph.

In the previous example, Dehumidifier A has 26 GPP of grain depression. This means that for every 14 cubic feet of air that is moving through the dehumidifier, 26/7000ths of a pound of water is dropping into the collection pan. Every minute, 320 cubic feet of air move, which computes to 594 grains of water removed per minute. There are 1,440 minutes in a day, therefore 855,772 grains of water will be removed in a day (if 26 grain depression keeps up). Converting grains to pounds, 122 pounds of water will be removed. Since there are 8.34 pounds in a gallon, there would be 14.7 gallons removed. Eight pints per gallon calculates out to just over 117 pints of water removed.

Estimating Pint Removal: For the Rest of Us

In order to estimate pint removal on a daily basis for your dehumidifiers simply calculate grain depression and combine it in this formula:

Using this formula allows an estimate of pint removal within 1/10th of a pint (in Nashville that’s what we call “close enough!”). As you can see in Chart 2, with a pint removal estimate it is easy to determine how much water dehumidifiers are removing and make better decisions on the job.

Graph 1

Determining Actual Pint Removal

Just a reminder, the previous formula is an estimation. A dehumidifier’s performance may go up or down throughout a day. Dehumidifiers have especially variable readings when they are in defrost mode. Graph 1 shows the dehumidifier output – in GPP – of a hot-gas bypass equipped dehumidifier while it is going in and out of defrost cycles in a 24-hour period.

As you can see, depending on when the reading was taken, grain depression could vary widely. The only way to find exact pint removal from a dehumidifier is to collect the water in a bucket, but I don’t personally recommend this because I’ve seen too many buckets fail.

Graph 2

Conventional vs. LGR

Conventional and low-grain refrigerant (LGR) dehumidifiers both work on similar principles. Air that is being pulled though the dehumidifier is cooled below dew point. The further the air is cooled below dew point, the more water is removed.

The factor that has the greatest effect on dehumidifier performance is the difference between the dew point of the intake air and the temperature of the intake air. This factor indicates how much work the dehumidifier is going to have to do to remove water. The more the difference, the less water will be removed, as illustrated in Chart 3 and Graph 2. The result of this is that as the air becomes drier, dehumidifiers remove less water.

A low-grain refrigerant overcomes this problem by adding additional cooling power to the system. Pre-cooling uses the magic of thermodynamics and to cool the air even more. LGRs pre-cool the air coming in to the dehumidifier; this pre-cooling uses virtually no electricity.

Chart 3

Doubling the Performance of Dehumidifiers

To summarize information thus far:
  • Dehumidifiers work better when the intake air temperature is closer to dew point.
  • Low-grain refrigerant dehumidifiers “pre-cool” the air.

As the air in a structure dries throughout the job, dehumidifiers have to work harder to remove water. Water removal declines as the job goes on. Water removal can be doubled simply by pre-cooling the air even more before it enters the dehumidifier. You can achieve this by:
  • Placing the dehumidifier intake near an air conditioner output.
  • Ducting air conditioner output into the intake of the dehumidifier (especially useful with ceiling diffusers).
  • Using a commercially available dehumidifier amplifier.

In tests performed in our facility, intake air temperature can be decreased significantly by feeding the dehumidifier with air conditioned air, as Chart 4 shows. However, this setup must only be used when the air is relatively dry (below 60 GPP). Restorers will know that this opportunity exists when air conditioning systems are no longer removing moisture (no longer have a grain depression). If this setup is used when air conditioning systems are still removing water, then the air provided to the refrigerant will not only be cooler, it will be drier – and the drier air will limit dehumidifier performance.

Chart 4


As you approach every water loss, you can use this information to:
  • Prove that your dehumidifiers are actually doing something through grain depression.
  • Make informed decisions about equipment placement based on estimated pint removal.
  • Record and document better dehumidifier readings.
  • Double performance of dehumidifiers in drier conditions.

And last but not least: if you are wondering what the last 1,400-plus words were about, I encourage you to further your education in water restoration. Today’s water restoration courses spend more time explaining psychrometry and equipment placement. You’ll learn the basics, so you can catch on to more of the advanced psychrometry discussed here.

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