Perma Pure MD-R Series - Manual (Page 2)

(P
s
/2P
v
) – 1
PRINCIPLE OF OPERATION
NOTE: Pressure units must be in absolute
terms.
For proper operation, be sure that the
pressure ratio, P
s
/P
v
is greater than 2. Vacuum
pump on purge exhaust is optional.
Where:
Vp = Purge flowrate (indicated on flowmeter)
Vs = Sample flowrate (indicated on flowmeter)
Ps = Sample pressure (in absolute units)
Pv = Purge pressure (in absolute units)
V
p
= V
s
1. STANDARD METHOD 2. REFLUX METHOD 3. SPLIT SAMPLE METHOD
The reflux setup, shown in Figure 2, returns
dry sample gas back to the dryer for use as
the purge after it has gone through the
analyzer. Since this method uses all of the
dry sample gas as the purge gas, only the
sample gas flow required for analysis
passes through the dryer. This results in
high drying efficiency
.
The vacuum on the purge gas should be at
least 15” of Hg, with a higher vacuum
preferable. This vacuum level is necessary
to provide the desired 2:1 purge-to-sample
gas flow ratio.
The split sample method, shown in Figure 3,
diverts some of the sample from the main
stream to be used as the purge gas. More
sample gas passes through the dryer than is
required for the analysis, lowering the drying
efficiency when compared to the reflux
method. The following equation can be used
to determine the purge flow rate required for
the split sample method. Any units may be
used as long as they are consistent. MD-
R™ Series gas dryers are shell and tube
moisture exchangers that transfer water
vapor between two countercurrent flowing
gas streams. The dryers consist of a Nafion
®
polymer tube surrounded by an outer tube.
Dry purge gas flowing over the exterior
surface of the Nafion tubing continuously ex-
tracts water vapor from the gas stream inside
the tubing. The driving force is the difference
in water concentration on the opposite sides
of the tubing wall. The purge gas then
carries
the water vapor away.
The most efficient way to set up MD-R
Series dryers is to have sample gas enter
through the Nafion tube (wet sample gas
inlet) and purge gas flowing countercurrent
to the sample (refer to Figure 1). Purge
gas should be instrument air or other dry
gas. If no dry purge gas is available, the
reflux or split sample purging method
shown to the right may be used.
INSTALLATION SPECIFICATIONS
When installing MD-R Series gas dryers,
the following rules apply:
1. Sample pressure equal to or greater
than purge pressure.
2. 2. Sample gas pressure not to exceed
80 psig.
3. 3. Temperatures must not exceed
120°C.
4. 4. Purge air of -40°C dew point at a
flow rate of approximately two to times
sample flow is required to achieve
rated performance.
5. Sample and purge air must flow
counter-current to each other.
6. If sample dew point is above ambient
temperature, inlet of dryer must be
heated. Call factory for details.
Figure 1 - Typical Setup
Figure 2 - Reflux Setup
Figure 3 – Split Sample Setup