32 36 Keep Your Cool JD
32
Keep Your
producingresults
By W.B. “Bud” Graham,
Improve performance without overheating
Contributing Writer
E
heat causes lubricants to thin and
tube production facilities have sud-
lose their lubricity, which causes
denly been restored to cool and effi-
verybody pushes their equip-
seals, bearings, and gears to burn up.
cient production after initiating good
ment. Every production manager
Can you hold your hand on the gear-
housekeeping habits.
wants more: faster production, higher
box, hydraulic valve, hydraulic reser-
2. Clean every heat exchanger’s
speeds, heavier gauge, higher
voir, or motor comfortably? If you
internal passages. Heat exchangers
strengths. You know the mantra: Give
can’t, take action!
can become clogged with sediment
me more! This is usually accompanied
Tips for Battling Heat. Several
or lime deposits if the cooling water
by very specific budget information:
tactics can help you keep heat under
is not properly maintained. Use a
We won’t give you more!
control.
cleaning rod to clean tube units or
A typical question is, How do I
1. Every tube mill has numerous
disassemble plate-and-fin units to get
size a heat exchanger to reduce the
heat exchangers—that is, surfaces that
rid of the buildup so flow is restored
mill gearbox oil temperature?
dissipate heat. Clean them. This
and heat can be absorbed and trans-
Although this is a specific question,
means mill bases, gearboxes (espe-
ferred. Sediment and lime are insula-
it’s part of a more general strategy:
cially cooling fins), hydraulic tanks,
tors; they don’t transfer heat well.
getting more work out of existing
piping, valve body manifolds, electric
3. If you have oil-to-air heat
equipment. Implementing this strate-
motor air inlets and outlets, and elec-
exchangers, add cooling fans to
gy means that much of the equip-
trical cabinets. Hydraulic tanks
increase heat exchange efficiency.
ment will run hotter than it did
should be off the floor so air can cir-
4. Switch to higher-temperature-
before. You can’t do much about
culate around all sides and the bot-
tolerant lubricants.
management’s insistence to get more
tom. Get rid of the buildup of scale,
5. Increase the ∆ T (difference in
out without putting more in, but you
dirt, dust, oil, grease, and filth. These
temperature between two conditions)
can develop a plan so you—and the
surfaces should be so clean you can
of the working fluid versus the cooling
equipment—don’t have to take so
see your reflection on them. Comb
fluid (whether its air or liquid) to
much heat.
condenser fins to restore air pas-
extract the maximum amount of heat.
sages, and flush them with the proper
Consider alternatives to the typical
Too Hot to Handle
solvents to eliminate oil and dirt
cooling tower, especially if your facility
The No. 1 enemy of machine com-
coatings. Replace air filters on elec-
is in an extremely hot or humid cli-
ponents such as gearboxes, hydraulic
tric motors and cabinets, and main-
mate. Refrigerated cooling can pro-
pumps, and valves is heat. When we
tain ductwork so cooling air goes
duce 40-degree-F chilled water rather
work out we want to feel the burn.
where it is supposed to go.
than the ambient-temperature water
This builds muscles. In your mill
It sounds simple, and it is, but it’s
that a cooling tower provides, which
gearboxes or hydraulic system, excess
an important first step. In fact, some
can be 90 degrees F or higher.
June 2004 tpj
A TPA Publication
33
producingresults
6. Try to maintain the turbulent
lowest water flow rate. It costs money
minus 855 BTU per hour = 1,616
flow of the cooling stream over the
to pump and treat the water, so a
BTU per hour. This is the heat load
hot surface to maximize heat rejec-
lower flow rate is better than a higher
per gearbox the new heat exchanger
tion. Laminar flow (smooth, continu-
one for overall plant efficiency.
must absorb to reduce the overall
ous airflow) creates boundary layers
operating temperature to 120 degrees
Sizing the Heat Exchanger
that slow heat transfer.
F. The total heat exchange for the
7. Add heat exchanger capacity to
OK, you tried everything men-
four gearboxes is 6,464 BTU per
handle the heat load.
tioned previously but the gearbox
hour.
Strategies That Can Backfire.
temperature is still pushing 200
7. To convert BTU per hour to
Some seemingly logical approaches
degree F or more. What do you do?
horsepower, divide the quantity of
can actually cause a greater heat
For this example, we will consider
BTUs by 2,545. For our example, this
buildup.
the sizing section of the mill as the
is 1,616 divided by 2,545 = 0.63
Be careful not to increase the flow
candidate for additional cooling. Our
horsepower per gearbox, or 2.5
of work or cooling fluids to the heat
example has four sizing passes.
horsepower in total.
exchanger to the point of diminish-
1. Measure the air temperature in
Some Like It Hot
ing return. Opening the water flow
the room far enough away from the
valves all the way to increase coolant
mill so that the gearbox temperatures
This example is a simplification of
flow actually may reduce the amount
do not affect the reading. Let’s use
the overall problem in that it assumes
of cooling. Imagine walking slowly
75 degrees F for the air temperature.
that all will be well when the gearbox-
past an air conditioner on a hot day,
2. Measure the average tempera-
es are operating at the desired 120
or racing past it. If you race past it,
ture of the gearboxes. We’ll say it’s
degrees F. The reduction in tempera-
you won’t cool down much. It’s the
205 degrees F.
ture will extend the bearing and gear
same with coolant flow in a heat
3. Estimate the surface area of a
life, but this doesn’t eliminate the
exchanger. To get the best perfor-
single gearbox in square feet. For this
cause of the extra heat in the first
mance from a heat exchanger, you
example, it is 7.47 sq. ft. per gearbox.
place, which is the mechanical over-
need to know the flow rate of the
4. Calculate the heat radiating
loading of the system. Like the weak-
work fluids and cooling fluids and
from the gearbox reducer in British
est link in a chain, eventually some
their change in temperature after
thermal units (BTU) per hour. The
component—a bearing, gear, valve,
passing through the heat exchanger.
technical term for this is the current
pump, or motor—will succumb to the
Every fluid, whether it’s water radiant load. Use this formula:
heat and fail. Eliminating the extra
or oil, and air, should be monitored
BTU per hour = 2.545 ✕ Area ✕
heat only extends the components’
by a flowmeter and temperature
TD (temperature difference between
useful lifetimes.
the air and the gearbox)
probes to measure the transfer rates.
But that’s the nature of the tube
Therefore,
Oil should enter the heat exchanger
producing industry. It’s competitive,
BTU per hour = 2.545 ✕ 7.47 ✕
at less than 160 degrees F and exit
and tube mills have to run fast—and
(205 – 75) = 2,471 BTU per hour per
the heat exchanger at or below
hot—to remain viable.
gearbox.
140 degrees F. This achieves a mini-
5. Do the same calculation for the
Bud Graham is president of Welded
mum 20-degree ∆ T (change in tem-
desired operating temperature for the
Tube Pros, P.O. Box 202,
perature).
gearbox. Let’s use 120 degrees F. The
Doylestown, OH 44230, 330-
For tube-type heat exchangers, the
desired radiant load at this tempera-
658-7070, fax 312-896-5696,
water flow should be about one-half
ture would be
budg@bright.net, www.weldedtube
pros.com. He also is the chairman of
that of the oil and not hotter than 90
2.545 ✕ 7.47 ✕ (120 – 75) = 855.5
TPA’s Tube Producers Council. If
degrees F at entry. Flow rates for oil
BTU per hour per gearbox
you have a specific question or
and water should be between 2 and 5
6. The difference between the cur-
would like to see an article on a par-
feet per second. The ideal balance
rent radiant load and the desired
ticular problem, please contact the
extracts the maximum heat with the
load is therefore 2,471 BTU per hour
author or TPJ.
tpj June 2004
A TPA Publication
