controlled by a cam timer, which may be electric (stan-
dard) or pneumatic (optional). The schematic diagram,
Figure 2, shows the airflow through the dryer with ves-
sel A on stream and vessel B being reactivated.
With purge valve E closed, incoming wet air flows
through shuttle valve S into the bottom of vessel A;
moisture is removed from the air stream by the
desiccant. Dry air exits the top of the vessel and divides
into two streams: process air exits through the shuttle
valve T, and a portion of the dried air (purge air) is me-
tered through orifice C and reduced to atmospheric
pressure by orifice D, greatly expanding its volume. The
expanded purge air flows down through vessel B and
out through the purge valve F and muffler, carrying
away moisture adsorbed by the desiccant in the previous
cycle.
The dryer operates this way for approximately 4 min-
utes. Then purge valve F closes, allowing vessel B to
gradually repressurize for approximately 1 minute be-
fore the cycle changes. The cycle timing sequence will
vary depending on operating pressure. At switchover,
purge valve E opens to depressurize vessel A. This
causes shuttle valves S and T to shift so that vessel B is
on stream and vessel A is being reactivated.
Start-Up
Once the HRD Series heatless dryer has been installed
according to instructions, it is ready to be operated. The
dryer is fully automatic and does not require any auxil-
iary controls.
To start the dryer:
1. Close the customer-supplied letdown valve and the
shutoff valve downstream of the dryer and
afterfilter.
2. Open the customer-supplied bypass valve.
3. Slowly open the customer-supplied shutoff valve
upstream of the dryer and prefilter to pressurize the
dryer until both vessels reach line pressure.
4. Turn on the power switch located on the control en-
closure. Depending on the position of the switching
valve and specified operating pressure, one of the
vessels will depressurize, or both vessels will re-
main pressurized for up to 90 seconds, then one
will depressurize.
CAUTION
If the dryer is equipped with optional pneu-
matic control, check the pressure gauges in-
side the control enclosure. Correct control-air
operating pressure is indicated below the
gauges. Adjust the pressure regulators as re-
quired.
5. Operate the dryer for three cycles to verify satisfac-
tory operation. The dryer cycle is 10 minutes, with
each vessel on stream for five minutes.
6. Slowly open the outlet shutoff valve.
7. Close the dryer bypass valve.
Dew Point
The pressure dew point of the outlet air is controlled by
the amount of purge air used for reactivation. Purge air-
flow is continuous and is adjustable within a set range.
Orifices in the purge air line are sized to control to the
highest dew point (–20°F). Lower dew points can be se-
lected by manually adjusting the purge adjustment valve
to increase the purge airflow. Dryer installation and
start-up must be completed before the purge valve is ad-
justed.
Standard HRD Series heatless dryers are designed for
pressure dew points between –20°F and –60°F. For ap-
plications requiring dew points outside this range con-
tact your local distributor.
Dew Point Adjustment
After your dryer has been installed and the start-up pro-
cedure completed, adjust the dryer for a –40°F dew
point as follows.
1. Determine air pressure (psig) at the dryer inlet.
2. Refer to Table 3. For your model and inlet air pres-
sure, determine the inlet airflow and average purge
rate (% of inlet air) required to produce a –40°F
dew point.
3. Calculate the required purge flow (scfm):
Purge Flow = Purge Rate x Inlet Flow
4. Refer to the purge flow calibration graph shipped
with the dryer. On the graph, locate the average
6 HRD Series Heatless Dryer (Bulletin 341)