Condition Monitoring Air Coolers

Condition Monitoring of Air Cooled
Heat Exchangers
R J BERRYMAN
Heat Exchanger Advisory Service

AEA Technology
Harwell Laboratory
Oxfordshire
United Kingdom
Summary
The efficient and optimised operation of Air Cooled Heat Exchangers is vital in all Petro-
chemical and Refinery operations where low grade heat is released to the environment. The
need for continuous operation of these exchangers has often discouraged routine condition
monitoring with the inevitable fall-off in performance with time. This paper explains how
these exchangers can be performance tested and describes instruments, test methods, data
analysis and diagnostic procedures used in maintaining and improving performance. The
use of condition monitoring and good design and installation practices can lead to financial
benefits with very short pay-back periods.
Background
Air Cooled Heat Exchangers (ACHEs) are used extensively on Oil Refineries and Petro-
chemical complexes for condensing and cooling numerous process streams. The efficiency
of such units for the rejection of low grade heat to the atmosphere depend a very great deal
upon the thermal and mechanical performance of the tube bundle and fans. Optimisation is
carried out in the initial design stage to minimise the heat transfer surface and provide an
exchanger which can cope with the original process stream.
Since ACHE performance is subject to climatic conditions, they can become the cause of
"bottle-necks" in overall plant throughput particularly during times of high ambient air tem-
perature. Such events have become commonplace on many process plants in Europe dur-
ing recent years and have led to production departments looking more closely at the
parameters which effect ACHE performance.
The Heat Exchanger Advisory Service (HEAS), which is a trouble-shooting and consultancy
organisation, has carried out many evaluations on existing ACHE units in recent times. Set
up some six years ago with technical support from the Heat Transfer and Fluid Flow Service
W. Roetzel et al. (eds.), Design and Operation of Heat Exchangers

© Springer-Verlag Berlin Heidelberg 1992
211
(HTFS), the Advisory Service uses methods and techniques developed from many years
research into ACHE thermal and mechanical problems. Berryman and Russell (1985)
describe various aspects of trouble-shooting and include specific case histories.
This paper describes the techniques and instrumentation used to evaluate the performance
of an ACHE both initially and after any remedial steps have been taken to improve the situ-
ation. The reasons for possible loss In performance are discussed as are methods which
can be used to improve or uprate an existing unit.
Also covered are the important basic criteria which should be considered when designing
new units. This includes thermal, mechanical and aerodynamic aspects for the optimisation
of ACHE performance.
By the application of on-line condition monitoring it is possible to increase the performance

of most ACHEs by as much as 25%. This applies especially to older units which have not
been regularly cleaned and/or where less efficient fans have been used. The use of modern
computer software for the design and performance simulation of existing units has proved
valuable in getting the best out of air cooled heat exchangers.
The financial benefit to the user of having ACHes working at peak performance can be very
large with the pay-back period on the return of money invested in performance testing being
a matter of just a few days.
Performance ~
The essential operating data required in order to assess the performance of ACHEs is
usually obtained from the airside. By far the most important parameter Is airflow and its tem-
perature at inlet and outlet. Most exchangers are of the forced draught design with axial
fans driving cold air into plenum (box) chambers which support extended surface, finned
tube bundles. Measurement of air inlet temperature is easily made at 5 locations below the
fan. Any noticeable difference between this temperature and the surrounding ambient value
usually indicates hot air recirculation or radiant heating from adjacent pipework or vessels.
At exchanger outlet, i.e. above the tube bundle in forced draught designs, air velocity and
temperature is measured simultaneously at multiple locations. The bundle is divided into
known flow areas and measurements are made at 150 mm above the finned tube surface.
At this distance any effects of high velocity "jetting" between the fins is minimised. About 30
determinations are usually made over a tube bundle measuring, typically, 2 metres x
9 metres. Two such bundles are normally served by two fans (3.66 m diameter) and the
data on airflow and temperature rise are all that is required to determine the airside heat
load. Berryman and Russell (1986) give further details on how to measure airflow in ACHEs
including comparisons using different instruments.

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Condition Monitoring of Air Cooled

Heat Exchanger Advisory Service

W. Roetzel et al. (eds.), Design and Operation of Heat Exchangers

By the application of on-line condition monitoring it is possible to increase the performance

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