Tips on Selection of Shell and Tube Heat Exchangers
FLUID POWER - Design Data Sheet 46
The reader may want to refer to Design Data
Sheet #1 for formula relating to heat transfer and for
additional information on heat exchanger selection.
Information in this issue relates only to industrial hydraulic
systems using mineral base oil and using water as a cooling medium.
The task of selecting an appropriate size heat exchanger for this
type of application is quite easy after a few basic facts on heat
and heat transfer are learned.
Catalog selection data published by manufacturers of heat
exchangers would lead a person to believe it is a highly
complicated problem involving precise mathematical calculations.
This is true, of course, when calculating the exact rating of heat
exchanger models for catalog purposes. But from a practical
standpoint these precise calculations are of little value because
rarely is it possible to arrive at an exact amount of heat which
must be removed from the hydraulic system. Therefore, if the data
used in the mathematical equations are incorrect, the equations
will not produce an accurate answer.
Any fluid power designer, by using a fair amount of common
sense, can come up with a better selection than can a computer if
that computer is fed with incorrect information. Here are a few
facts about heat transfer:
Measurement of Heat Load. In hydraulic systems
it is usually more convenient to express heat losses and heat
transfer in HP rather than BTU/minute because other system
calculations involving power are expressed in HP. Design Data
Sheet #1 gives conversions into HP from kilowatts,
Levelling-Off Temperature. In any hydraulic
system, when the system is first started, temperature of oil in the
reservoir will gradually rise from room temperature (ambient) to a
"levelling-off" temperature in a period of time which may vary from
15 minutes to several hours, depending on the type of system, the
duty cycle, and its natural heat radiation capability. After this,
no matter how long the system remains in operation, the oil
temperature will rise no higher unless there should be a change
either in the duty cycle or in the ambient temperature. At this
temperature a balance has been reached between the rate at which
heat is generated and the rate at which the system can dispose of
it by natural radiation cooling. If this levelling-off temperature
is low enough to be acceptable, no heat exchanger is needed. The
purpose of a heat exchanger is simply to reduce the leveling off
temperature to an acceptable level. Design Data
Sheet #1 gives information on a procedure for finding
heat exchanger capacity to supplement an existing system.
Shell and Tube Heat
Exchanger. Consists of a bundle
of brass or naval bronze tubes inside an outer
and enclosed by a pair of end caps. Cooling water
circulated through the tubes; hot oil is circulated
the shell and around the outside of the tubes.
Natural Radiation Cooling. Heat is radiated
from all metal parts of a hydraulic system including oil reservoir,
cylinder, hydraulic motor, valves, and from iron and steel fittings
and plumbing. On many low power systems the natural radiation is
adequate to keep the levelling-off temperature to an acceptable
level, especially where the duty cycle is short. But on larger
systems the area of the heat radiating surfaces is far less in
proportion to the system power. On systems larger than 25 HP, and
with long duty cycle, a heat exchanger is usually necessary to
avoid a high levelling-off temperature.
Estimating Heat Load. Before any heat exchanger
can be intelligently selected, an estimate must be made of the
probable system losses. Most of these losses will turn up as heat,
which must be disposed of by a heat exchanger with the help of
natural radiation. It is seldom that these losses can be calculated
accurately. Therefore, the designer has to rely a lot on common
sense, and should select a heat exchanger which he is sure will
have more than sufficient capacity. An oversize heat exchanger,
while its first cost is higher, will not consume any more cooling
water than a smaller unit. The water control valve will always
meter in only enough water to match the heat load whatever the size
of the heat exchanger. A "common sense" method of estimating the
system heat load is described next:
Maximum Heat Exchanger Capacity. Common sense
tells us that the heat exchanger capacity will never have to be
greater than the HP input even if the entire input was going into
heat. In all except very unusual systems, the power losses (heat to
be removed) will be less than 50% of the input power and usually
will be more like 25%. This is a very rough way of estimating heat
load. Sometimes the heat load can be more accurately estimated as
Heat Generation in the System. Each hydraulic
pump or motor can be expected to waste about 15% of its input power
through internal slippage, and most of this ends up as heat in the
oil. Additional heat comes from flow of oil across relief valves,
flow control valves, and pressure reducing valves. This is
calculated with the fluid power formula:
HP = GPM × PSI ÷
In a properly plumbed system it has been estimated that about
10% to 15% of the input power will go into heat from flow losses
through plumbing, fittings, and directional control valves. If all
of these losses are calculated and averaged over a period of about
an hour, a heat exchanger having this capacity will be more than
Cooling Water. Our rule-of-thumb (see box on
front Of this sheet) requires an available flow of water of 1/2 the
oil flow rate. When using the modulating type water control
described in the next paragraph, the water will not be flowing
continuously at this rate. During start-up and probably for several
hours, no water at all may flow. Then, the actual flow rate
thereafter will be only that amount needed to carry away the heat,
to maintain the levelling-off temperature set on the water control.
Thus the actual flow, averaged over a day's operation may be only a
small fraction of the volume provided for maximum heat removal
Water Control Valve. A modulating type water
control valve should be installed upstream of the heat exchanger.
This valve has a temperature sensing bulb connected by a 6-foot
long capillary tube into the main valve. The bulb must be immersed
in the hydraulic oil. Usually the reservoir is the most convenient
location for immersion, although any part of the system may be used
for temperature sensing. The control is then adjusted for the
desired leveling-off temperature. Water remains completely cut off
until system temperature approaches the setting of the control
valve. The valve then modulates the water flow, allowing only
enough flow to keep oil temperature from rising any further.
We do not recommend a 2-way solenoid water valve controlled by a
temperature switch, as this method tends to waste water.
The bulb of the water control
valve must be
immersed in the hydraulic oil.
Volume of Oil Flow. An oil flow rate near the
mid range of the heat exchanger flow rating gives optimum
performance. At higher oil flows more heat will be transferred but
there will be much higher flow losses through the heat exchanger
and more back pressure on the system. At lower flows the heat
removal capacity is reduced.
How to Apply the Rule-of-Thumb
The rule of thumb in the box on the front side of this sheet is
based on an available volume of cooling water of 1/2 GPM or more of
water for each 1 GPM of oil flow, and at a temperature no higher
than 90°F. Reservoir temperature will be held below 150°F provided
the ambient (room) temperature does not exceed 100°F.
- You should first estimate, as realistically as you can, the
amount of HP in heat which will have to be disposed of through the
heat exchanger. Use the information in the opposite column and on
the front side of this sheet as a guide.
- You must know the volume of oil, in GPM, which will flow
through the heat exchanger when installed in the circuit position
you have chosen.
- The heat exchanger catalog from which you are making a
selection must show the square foot transfer area of each model,
and must show the minimum and maximum flow limits, on the oil side,
for each model.
- Select a model for which your GPM oil flow falls in the 1/3rd
to 2/3rds range between the minimum and maximum mum flow limits
given for that model.
- Use the rule-of-thumb to see if the chosen model will transfer
the amount of heat which you have estimated for your system. If it
will not, select the next larger model.
More information will be presented in future issues on selection
of air cooled heat exchangers, and on installation and maintenance
of all heat exchangers.
Download a PDF of
Fluid Power Design Data Sheet 46 - Tips on Selection of Shell and
Tube Heat Exchangers.
© 1990 by Womack Machine Supply Co. This
company assumes no liability for errors in data nor in safe and/or
satisfactory operation of equipment designed from this