*TM 5-810-4/AFM 88-8, Chap. 3
CHAPTER 6
AIR DRYER
6-1.
Application.
purge air to regenerate the offstream tower. By reducing the
amount of purge air required for regeneration, the heat re-
Some compressed air applications require moisture removal
generative dryer operating costs are lower. High regenerative
in addition to that provided by an aftercooler. Such applica-
temperatures, however, are damaging to equipment and de-
tions include paint spraying, sandblasting, use of air-oper-
siccant, so any savings in operating costs can be outweighed
ated tools and devices, pneumatic automatic temperature
by the costs of maintenance and downtime.
controls, lines run outside in cold or subfreezing locations,
c. Deliquescent. Deliquescent (salt pellet) dryers and
and lines passing through cold storage rooms.
ethylene glycol stills are included in this manual for compari-
son and general information purposes only, and will not be
6-2.
Dryer types.
used because of their high operating cost and their limited
effect on pressure dew point. These types of dryers carry over
Supplementary moisture removal requires additional equip-
salt or glycol into the airlines, resulting in corrosion and
ment, higher first cost, and higher operating cost for all
potential damage to controls and tools. Glycol also reacts
drying methods. In determining overall costs, the initial pur-
with certain constituents of the air (mainly carbon dioxide
chase price should be weighed against operating and mainte-
and carbon monoxide) to form corrosive compounds that
nance costs. Figure 6-I illustrates the relative costs for the
attack piping and equipment.
various types of dryers, and presents selection guidelines. In
determining the type of dryer to be used for a given
6.3.
Prefilters and afterfilters.
application, drying requirements, flow, pressure, inlet tem-
peratures, and the pressure dew point must be accurately
Consideration should be given to providing a prefilter up-
determined. The dryer that meets these requirements most
stream of the air dryer and an afterfilter downstream of the
economically and efficiently should be selected. The various
air dryer. A prefilter may be required to remove compressor
drying methods are as follows:
carry-over oil and other undesirable particles from the air
a. Refrigeration. Refrigeration dryers remove moisture
prior to the air entering the air dryer. This filter can extend
from compressed air by cooling the air in a heat exchanger.
the life of the air dryer and reduce air dryer maintenance
This condenses and removes the moisture from the airstream
costs. An afterfilter should be considered to protect the
and produces an operating pressure dew point at the dryer
downstream piping system and equipment from impurities
outlet in the range of 35 to 39 degrees F. By adjusting the
and undesirable particles added to the air as a result of
refrigeration unit operating parameters, these units can pro-
passing through the air dryer. Air dryer manufacturers should
duce pressure dew points of 50 degrees F. Higher dew points
be consulted for recommendations and selection of prefilters
are available in either direct refrigeration or chiller-type
and after filters for specific air quality requirements.
design.
b. Twin-tower regenerative. Regenerative dryers utilize
nonconsumable desiccants to remove moisture from com-
pressed air. Inlet air is automatically cycled between two
desiccant towers, one absorbing moisture from the inlet air
while the other is being regenerated. This method of regen-
eration includes the following dryer classifications:
(1) Heatless desiccant regeneration passes a quantity of
dried (purge) air through the offstream bed. No external heat
is applied. This type, with a field-adjustable purge control
should be selected so that purge rate (and therefor pressure
dew point) can be adjusted to accomodate seasonal
variations in ambient temperatures, thereby reducing operat-
ing costs. Heatless dryers are capable of providing minus
150 degrees F, pressure dew point. Maintenance costs are
low since there are few* moving parts. With adequate prefil-
tering to remove oil, desiccant replacement requirements are
minimal.
(2) Heat regenerative dryers utilize heat from an
external source (either electric or steam) in conjunction with
6-1
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