After several journeys through the hot, humid, summer weather, the plant complains to their dryer provider. “That dryer you sold us is no good – we have water throughout the system.  It gets worse in the summer!  We keep having refrigeration compressor problems, etc.”  Often the answer or collective decision is – “since the refrigerated dryer doesn’t work – we need to install a desiccant dryer – that will give you a -40 degree F pressure dewpoint – it will take out “a lot more water.”

A new twin tower desiccant dryer is installed which, has a higher installation and operating cost than the original refrigerated unit.  It is also rated for 1000 scfm at 100 psig and 100 degree F ambient temperature and 100 degree F inlet air temperature.

After several more journeys through the hot summer months, plant personnel realize they still have the same “water problems” and are starting to have more and more maintenance issues with the new desiccant dryer. – WHAT TO DO? – Higher cost – same results!

Trouble Shooting the System!

• “What is the measured pressure dewpoint?” ..."Don’t know, we didn’t install a dewpoint monitor – the one on the dryer is broken”

• “What is the temperature of the air entering the dryer?”...“Don’t know exactly – but it seems ok”

• “What is the pressure entering the dryer?”...“Don’t know the gauge is bad!”

The Forgotten Factor – The air cooled aftercooler on the compressor. The air cooled
aftercooler will have a designed ∆T or approach temperature when clean and the air from the
fan is flowing freely.  Reviewing the engineering specification sheets of various units, these
ratings run from 10 degree F approach to 20 degree F approach.

What this means is the aftercooler when rated with at “15 degree F approach” is that the
compressed air will be cooled to 15 degree F higher THAN THE TEMPERATURE OF THE
COOLING AIR THAT ENTER THE COOLER CORE.  This can never be lower than the
ambient air temperature and in all probability will be 15 degree F or higher depending on
cooler location – package design – surrounding equipment, etc.

For this exercise – let’s use a 15 degree F rated approach with an actual 20 degree F approach to ambient temperature.

Both the refrigerated and desiccant dryers are rated for flow at 100 degree F inlet air temperature.  This means in the case of the refrigerated dryer there is enough refrigeration capacity in Btu/hr to condense the total volume of water vapor held in gas form in 1000 scfm of air at 100 degree F, 100 psig.  In the case of the desiccant dryer, the applied height and volume of desiccant offers the proper surface area to adsorb approximately the same amount of water vapor.

What is the expected air cooled aftercooler delivered air temperature to the dryer during a 100 degree F ambient temperature day?  How about an 110 degree F ambient temperature day?

•100 degree F + 20 degree F approach = 120 degree F

•110 degree F + 30 degree F approach = 130 degree F

There is absolutely NO POSSIBLE WAY for an air cooled aftercooler to deliver 100 degree F air to the dryer at 100 degree F or higher ambient temperature.  In the real world with potential dirty coolers, fans, restricted flows, recirculation, etc. the net temperature will be higher.

Almost anywhere in the industrial areas of North America, the systems will see operating ambient temperatures of 90 degree F to 100 degree F.  This means the 100 degree F target inlet temperature is impossible to meet and condensate excursions into the system will occur. Their effect will depend on system design, layout, etc.

What happens when the pressure dewpoint delivered to the dryer is 120 degree F:

•The amount of water vapor delivered to the dryer
exceeds the available Btu/hr refrigeration available to
condense the vapor to liquid.  There is a great deal of the
condensate that stays in gas form increasing the pressure
dewpoint of the air going to the air system. (See figure 4).
The calculated pressure dewpoint at 120 degree F inlet
compressed air to 100oF inlet increases from +40 degree F to
+77 degree F PDP.  As the air cools through the system,
liquid water drops out and settles in the system.  If it does not
blow out onto the process, it will lay in the low spots evaporating
and recontaminating any dry air that does enter the system.  

What action was taken in this example, the dryer was identified
as the root cause so a misapplied refrigerated dryer was replaced
with a more expensive “equally misapplied” desiccant dryer that
also didn’t perform!  We got equally dismal results at a probable
higher operating cost.

What are the options? The desiccant dryer in this scenario is
probably not a viable option because -40 degree F air has not
been identified as a need.  It too will be undersized with regard to
drying capacity, and more important if the extreme inlet air
temperature gets to above 130 degree F, the desiccant will stop any
effective adsorption of the water vapor and virtually no drying will occur.

If the desiccant dryer is to be used, then the best corrective action would be to install appropriately sized water cooled pre-cooler with separator before the dryer.  This will reduce the air temperature using some available cooling water supply.  It can be set up with a thermostatic controller shut off valve.  There are several options that can also be used as corrective action with refrigerated dryers or even better, considered when planning the system.

Options: With Refrigerated Dryers:

Live with the problem
Install a second inline refrigerated dryer / potential high pressure loss – extra energy cost
Water-cooled (well, chilled, etc.) Trim pre-cooler as described earlier.
Install larger dryer – double size or more:

When an extra water-cooled pre-cooler is not an option, over sizing the dryer, (double the rating), has some very significant benefits:

Lower pressure loss, proper selection can implement almost negligible pressure loss
Can still dry compressed air to an acceptable level even when extreme temperatures of 130oF are reached.
Some additional growth to the compressed air supply can be applied
Redundancy – some large flow dryers (primarily the cycling heat sink type) have independent parallel refrigeration internal systems, which are designed to be able to be valve out one refrigeration system for maintenance and repair.  This allows the remaining running parallel refrigeration system to continue to dry the air (called a MultiPlex design), thus creating built in redundancy and can preclude the requirement for a separate “backup” dryer for continuing drying.

Cycling – Non cycling Dryers: Doubling the refrigeration capacity to be able to handle the extreme temperatures may create some areas of increased stringent maintenance issues particularly when using a non-cycling dryer.

When using a non-cycling dryer, the Hot Gas By-pass valve will definitely require continuing seasonal adjustments to avoid “freeze up” from over cooling.  Since it will not cycle, the operating energy cost will be about twice the smaller dryer all year.  Some larger dryers may use refrigeration compressors with head unloading and even multiple units that shut off to reduce this.  Our observed field service has shown that this with out an effective heat sink to limit this action, short cycling will often occur causing premature refrigeration compressor failure.

A proper cycling dryer will eliminate any need for any seasonal adjustment to avoid overcooling freeze up.  The overcooling capability just creates more “off time” reducing the actual electrical energy cost.

Cycling dryers with proper heat sink capacity limiting the number of starts / stops of the refrigeration compressor to no more than 6 per hour, do very well in this application.

Figure 5 shows the effect of over sizing when:







































How much moisture or water vapor and or potential liquid condensate does an effective refrigeration dryer system deliver?

Figure 5 shows the effect of properly applied, sized, and operating aftercoolers combined with basic -40 degree F performance desiccant dryers and +38oF performing refrigerant dryers:

Entry air - .3122 gallons / 1000 cuft

After cooler air – .0478 gallons / 1000 cuft

Refrigerated dryer - .0058 gallons / 1000 cuft

•88% of the moisture load in the incoming air is removed with an effective 38 degree F rated refrigerated dryer.

SUMMARY:

Compressed Air Drying in High Ambient (over 100 degree F) with Air Cooled Equipment

Potential Problem: Sizing the Dryer Capacity about equal to Compressor Capacity

Avoid replacing “misapplied refrigerated dryers” with “equally misapplied regenerative desiccant dryers” – with equally dismal results at higher operating costs.

For more information click here.