Perma Pure MD-R User Manual

ELEMENT REPLACEMENT

PERFORMANCE

MD-R Series dryer’s performance varies according
to dryer lengths and flow rates.

Field replacement of MD dryer elements

longer than 48” is difficult without special

equipment and is not recommended. The

instructions below are for the 12”, 24” and 48”

length models

1. Hold the dryer end fitting assembly
behind the rotating purge fitting carrier.

2. Grip the end nut and rotate counter­clockwise to remove. Repeat for the
opposite end.

3. Gently pull the Nafion® tube element out
of the housing just far enough to be able
to remove the rubber ferrule. Remove the
rubber ferrule.

4. Remove the element by pulling the
element out from the other end of the
dryer housing.

5. When installing the new element, handle
the membrane tubing with gloves to
prevent contaminating the tubing surface.

6. Once the element is positioned with equal
lengths protruding from each fitting, install
the rubber ferrules about ¼” from the end.

7. Install the end nuts and tighten as much

as possible by hand.

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MD-R™ Series

Gas Dryer

User Manual

Figure 4

8 Executive Drive

Toms River, N.J. 08755

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Fax: [phone]

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Web Site: www.permapure.com

Performance Curves

PERMA PURE

Bulletin# 203R, p/n 102753, 7/11

Nafion® is a registered trademark of DuPont , MD™ is a trademark of Perma Pure LLC

(Ps/2Pv) – 1

PRINCIPLE OF OPERATION

NOTE: Pressure units must be in absolute
terms. For proper operation, be sure that the

pressure ratio, Ps/Pv 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)

Vp = Vs

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 ex­tracts 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