Over the years, many heaters have been returned as "defective." Unfortunately,
most of these heater failures were preventable. Preventive
maintenance is the key to longer heater life. Today's electric
immersion heaters are designed to regularly perform under harsh
conditions. You should expect good heater performance as long
as you are willing to take certain precautions. Here are examples
of typical failures and the preventive steps that should be
taken.
Improper electrical installation will result in poor performance.
Electric immersion heaters and temperature controls should be
properly wired according to code by a certified electrician.
All heater manufacturers provide wiring diagrams with every heater
or control shipped. Examine the tags and rating plates found
on the heater and control models to verify that they are the
correct kilowatts, voltage and amps. Include power surge protection
in your installation with the proper size fuse or circuit breaker.
Be careful to place metal immersion heaters in tank locations
where they will not contact outside power sources such as anodes.
Low liquid levels can lead to serious problems: heater failure
or tank fires. Today, heater manufacturers provide low-liquid-level
thermal shut-off fuses or sensors with electric immersion heaters.
Make sure that the proper heater thermal cut-off sensor is selected
in relation to tank operating temperature. Do not bypass thermal
fuse connections for expediency. Spare fuses should be kept in
supply. If you are going to use a resettable thermal cut-off
system, one that employs a bimetallic sensor, make sure your
electrician properly installs it. Do not try to mix different
brands of heaters and controls with resettable systems because
they may not be compatible (Fig. 1, below).
 |
Figure 1. Ensure that systems
are
properly installed. |
Always maintain at least two inches of liquid level above the
heater's hot zone to avoid over-temperature damage or thermal
sensor activation (Fig. 2, below). Make sure that the area between
the heater body and thermal sensor on the heater is kept free
of sediment. This could cause the sensor to improperly sense
heat and delay heating action. It is also a good idea to back
up your heater's thermal shut-off sensor with a liquid level
control. Do not rely solely on visual control as a method of
backup. People can and do make mistakes.
 |
Figure 2. Maintain two inches
of liquid above the heater's
hot zone. |
Build-up or scaling on immersion heaters is one of the leading
causes of heater failure. Electric immersion heaters generate
high temperatures. Build-up or scaling entraps the heat within
the heater and leads to burnout. You can see the dark discoloration
on the heater element and in some cases you will see actual holes
with melted metal around the perimeter (Fig. 3, below left).
 |
Figure 3. Build-up or scaling
is
a major cause of heater failure. |
|
 |
 |
Figure 4. Identical wattage
heater
models. |
|
Heaters should be cleaned regularly. Cleaning can be done
by scraping metal heaters, chemically dipping the heaters or
pressure spraying. Never try to chip off build-up with a hammer
since it may damage internal heater components. Another good
preventive step for build-up reduction is to use derated electric
immersion heaters. Derated heaters provide a cooler surface
and thus reduce the rate of build up that is occurring. The
heater's "hot
zone" is actually stretched to provide this cooler surface.
There is a trade off, since it may take more derated heaters
to do the same heating job of one standard watt density heater
because of physical size differences. For example, a derated
3kW heater will be the same length as a standard watt density
6 kW heater (Fig. 4, above right). Another cleaning tip is
to clean the bottom of the tank. Excessive sludge deposits
will eventually encapsulate the lower section of your heater's
hot zone trapping heat and causing a burnout. Regularly inspect
and remove all sediment and build- up to insure longer heater
operating life.
Chemical misapplication is another problem. Visual signs of
pitting and etching are the usual clues of a chemical attack.
I wish that I could simply say, "Don't put quartz heaters
in caustic baths and don't put steel heaters in acid baths." However,
it is not that simple, because many chemical processes have
different additives. Most electric immersion heater catalogs
provide a useful chemical application chart. This chart should
be considered a guideline. For best results, check with your
chemical supplier. He should supply you with a Material Safety
Data Sheet (MSDS). This sheet will provide information regarding
heater sheath material selection and other useful data. It
is usually not a good idea to move heaters from one type
of bath to another due to possible chemical differences.
Physical damage to quartz and Teflon® immersion heaters
is a concern. Quartz heaters can suffer breaks or cracks
from impact or stress. Teflon heaters are abrasive sensitive
in that they can be damaged by scrapes or cuts. Therefore,
these types of heaters should be carefully handled and a guard
should be used. Take care to position them so that they will
not come in contact with parts in process or with other tank
equipment.
Condensate entering the heater or control is yet another source
of concern. Do not mount heaters below the rim of the tank or
in an enclosed tank. Lifting a heater by its plastic armored
cable could damage it and cause a rupture where condensate will
enter. Try not to place the heaters by an exhaust duct or where
parts may drip on them. Make sure heaters remain well sealed
by inspecting them and re-sealing any loose connections with
silicon adhesive.
If you take these preventive steps to ensure longer heater life,
you will save both money and lost production time.
To obtain more information on products or processes from Cleveland
Process Corp., call (800) 241-0412.
