United States
Environmental Protection
Agency
Atmospheric Research and
Exposure Assessment Laboratory
Research Triangle Park, NC 27711
Research and Development
EPA/600/SR-93/229 February 1994
Project Summary
Closed Cycle Cooler for
VOC Preconcentration
Deborah L. Smith
The objective of this study was to
evaluate a reduced temperature precon-
centrator to replace or reduce liquid
cryogen use in field applications. The
cooler was to be evaluated as a refo-
cusing unit downstream of a solid sor-
bent trap. A closed cycle cooler from
Cryoworks (formerly Carlisle Cryotronics)
in Carlisle, MA, was selected for the
evaluation. Basic properties of the
cooler were evaluated, including cool-
down time and heating time. Modifica-
tions were made to improve the perfor-
mance of the unit during the testing
period. Details of the study are pre-
sented.
This Project Summary was developed
by EPA's Atmospheric Research and
Exposure Assessment Laboratory, Re-
search Triangle Park, NC, to announce
key findings of the research project
that is fully documented in a separate
report of the same title (see Project
Report ordering information at back).
Introduction
The use of automated gas chromato-
graphs at air quality network monitoring
stations to obtain concentrations of vola-
tile organic compounds (VOCs) has been
a long-term goal of the methods develop-
ment group at the U.S. Environmental Pro-
tection Agency. Developmental efforts by
EPA and other research laboratories have
resulted in several prototype units capable
of measuring VOCs in ambient air. These
prototype units generally utilize a solid
adsorbent trap to adsorb VOCs from an
air stream followed by thermal desorption
onto a liquid cryogen cooled refocusing
trap. The refocusing trap is in turn ther-
mally desorbed and the sample is sent to
a capillary column for resolution of the
components. In some instances the capil-
lary column is also subambiently cooled
to better facilitate compound resolution.
These automated gas chromatographs are
capable of measuring a wide range of
VOCs in ambient air that include C2
through C10 hydrocarbons, as well as se-
lect halogenated and oxygenated organ-
ics. To effectively retain and resolve the
C2 hydrocarbon species on a single col-
umn, a considerable amount of liquid cryo-
gen is required. Most systems cool the
adsorbent and the refocusing traps as well
as the analytical column by controlled re-
lease of liquid cryogen. The use of cryo-
gen becomes a significant disadvantage
when deploying the automated systems in
field monitoring studies. That is, the fre-
quent delivery of liquid cryogen required,
and the uneven quality of the product typi-
cally delivered, often result in manpower
commitments and, to a certain extent,
costs that detract from the usefulness of
the systems.
In an attempt to circumvent the use of
liquid cryogen, an alternative approach that
involved the use of a closed cycle cooling
device was investigated. Cryoworks (for-
merly Carlisle Cryotronics) designed and
produced a Gifford-McMahon style of
closed cycle cooler that incorporates me-
chanically operated valves to control gas
flow. Cryoworks had recently developed a
magnetic stroke technology in their cool-
ing engines for controlling the movement
of a gas driven, piston actuated displacer
(patent pending). Other commercial units
normally employ mechanically driven en-
gines. Increased operating efficiency as
well as enhanced long-term reliability were
key attributes of the Cryoworks technol-
ogy-
Printed on Recycled Paper
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Objective
The objective of this study was to evalu-
ate the Cryoworks* closed cycle cooler as
a refocusing unit downstream of a solid
adsorbent trap. Initial tests focused on
determining the system's capabilities in
terms of its cooling and heating efficiency.
Target time and temperature conditions
were determined based upon previous
work with automated refocusing units that
require liquid cryogen to retain C2 hydro-
carbons.
Evaluation
The Cryoworks closed cycle cooler was
evaluated as a refocusing unit downstream
of a solid adsorbent trap. Tests were per-
formed to determine the system's capa-
bilities in terms of cooling and heating
efficiency using target time and tempera-
ture conditions. For optimum performance
the Cryoworks cooler was required to
reach and maintain an initial temperature
of at least -180°C. The system was also
required to heat to 150°C within 30 sec-
onds. Finally the system was required to
return to -180°C within 20 min. Initial evalu-
ation included testing the system in its
original configuration as received from
Cryoworks. Further tests were performed
after modifications were made to the cold
station.
During Initial evaluation of the Cryoworks
closed cycle cooler the system was able
to reach -178°C after 2 hours but was not
able to maintain the temperature for any
extended period of time, indicating erratic
and unreliable behavior. Modifications were
made to the Cryoworks unit at Battelle to
improve its performance. These changes
resulted in allowing the cooler to be cycled
reliably to temperatures as low as -197°C
as well as the desired thermal cycling can
be repeated in as little as 14 min, well
within the requirements of operation of a
gas chromatograph for automated sam-
pling and analysis. Furthermore, the rise
time for heating was approximately 0.4
min, which also meets the requirements
for the closed cycle cooler. Unfortunately,
evidence of a small hotspot area was seen
just outside of the cold zone. The thermo-
couple was monitoring the temperature
near this area and was not giving a true
indication of the temperature within the
cold zone. Repeated attempts to improve
this situation were unsuccessful.
Conclusions and
Recommendations
In its original configuration from
Cryoworks, the closed cycle cooler was
unable to reach the temperature setpoint
of -180°C during initial cooldown. After
modifications were made by Battelle to
the cold station, the system was able to
reach -180°C in less than 90 min. Once
this temperature was achieved, thermal
cycling tests were performed to simulate
the use of the system in repetitive opera-
tion as an auto GC. During these tests,
the unit was able to reach approximately
150°C in 0.4 min and return to -180°C in
14 min. This is well within the target val-
ues set for the system. We believe that
during the initial test cycles the thermo-
couple was actually monitoring the tem-
perature near a hotspot and thus was
giving an erroneous indication of the tem-
perature within the cold head area. Efforts
to eliminate the hotspot and better control
the temperature were not successful.
The general impressions of the
Cryoworks closed cycle cooler are mostly
favorable. The system definitely has the
capability of cooling a section of fused
silica to -180°C and below, and we be-
lieve the system has additional capacity
to provide cooled air to reduce the tem-
perature of the primary sample collection
trap and the gas chromatographic column
of an automated system. The cooler can
be mounted easily onto a Hewlett-Packard
Model 5890 gas chromatograph directly
above the heated injection ports. This con-
figuration allows for a smooth transition
from the fused silica refocusing trap to the
analytical column.
We believe that the Cryoworks closed
cycle cooler would be a very effective
reduced temperature preconcentrator to
be used as a refocusing unit downstream
of a solid adsorbent trap. We have dem-
onstrated that the system meets the tar-
get time and temperature requirements
needed for cooling a section of fused silica
for collection of light hydrocarbons. We
recommend that further improvement to
the system is needed to optimize its per-
formance in the heating mode.
This report was submitted in fulfillment
of Contract No. 68-DO-0007 by Battelle's
Columbus Operations under the sponsor-
ship of the U.S. Environmental Protection
Agency. This report covers WA-7 and WA-
16 over a period from November 1990 to
August 1992, and work was completed as
of August 15,1992.
"Mention of trade names or commercial products does
not constitute endorsement or recommendation for
use.
£U.S. GOVERNMENT PRINTING OFFICE: HM - 55CHM7/MH2
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Deborah L Smith Is with Battelle, Columbus, OH 43201.
William A. McCtennyls the EPA Project Officer (see below).
The complete report, entitled "Closed Cycle Cooler for VOC Preconcentration,"
(Order No. PB94-130 390/AS; Cost: $17.50, subject to change) will be available
only from:
National Technical Information Service
5285 Port Royal Road
Springfield, VA 22161
Telephone: 703-487-4650
The EPA Project Officer can be contacted at:
Atmospheric Research and Exposure Assessment Laboratory
U.S. Environmental Protection Agency
Research Triangle Park, NC 27711
United States
Environmental Protection Agency
Center for Environmental Research Information
Cincinnati, OH 45268
BULK RATE
POSTAGE & FEES PAID
EPA
PERMIT No. G-35
Official Business
Penalty for Private Use $300
EPA/6QO/SR-93/229
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