Steam or Hot Water?
SX-2 Heating Coils
H. Otto Wolters, Owner of MBA Manufacturing, Inc.
The age old question for every plant manager is: "How do I provide heat to my operating processes?”
To answer this question the first decision is to what kind of source of energy to select. This often
becomes a decision between electricity, supplied by your local utilities, or the use of a boiler burning
fossil fuels such as oil, coal, wood or natural gas.
Electricity as source of energy can often be ruled out due to economic consideration as being too
expensive. Then fossil fuels usually become the energy of choice. In order to transform fossil fuel into
energy a boiler is required. Boilers are available in two basic styles: a) for hot water or b) for steam.
At this point in the decision process MBA's engineering department often becomes involved to provide
the technical support. What type of boiler to select: a boiler for hot water or steam?
It should be stressed that the energy required to heat a solution inside a tank from room temperature
to operating temperature will be the same regardless of the heating medium, hot water or steam.
Traditionally, submerged heating coils have been use to act as the heat transfer agent. These coils are
available in various forms to meet the individual requirements. For detailed information on shape and
sizes of heating coils, refer to MBA’s web site: www.mbamanufacturing.com
The efficiency of heat transfer from steam or hot water through coil walls into the tank solution is
determined by a combination of individual resistances. There is resistance to heat flow on the inside, as
well as, on the outside of the heating tube, as well as, resistance of the coil material itself. These three
resistances are combined in one factor, generally known as ‘U’ factor.
The formula to determine the necessary heat transfer for any application is: A = Q/ U x T
Where A is the required heat transfer area, Q = the amount of BTU’s necessary to achieve the desired
temperature rise of the solution inside the tank, U= the heat transfer coefficient, and T= the difference
between the start-up and the operating temperature. As can be seen, the larger the “U” factor, the
smaller the heat transfer area ‘A’, and consequently the size of the heating coil and vice versa.
LIQUID FLOW
As mentioned above, the heat transfer coefficient ‘U’ (under normal operating conditions for liquid
flow) consists of three factors, each resisting the smooth transfer of energy from the heating medium
to the liquid inside a tank.
The metal wall of the heating unit shows resistance to heat transfer; this resistance is usually very low,
since most metals are excellent heat conductors. This situation becomes quite different when plastic
heat exchangers are being used.
The other two subjects making up the U-factor are somewhat more complicated. They are the
insulating effects of moving or stationary liquids or gases (steam) on both surfaces: a. inside, and b.
outside, the heat transfer surfaces. These resistances can be quite substantial depending on the velocity
of liquid flow on either side of the heating units.
STEAM
When using steam instead of hot water as a heating medium the situation changes. Injecting steam
into a heat exchanger, two things happen. First, the friction factor of flowing water inside the heat
exchanger is eliminated, and secondly, the steam molecules condensate directly on the cooler walls on
the inside of the heat exchanger walls. The resistance to heat transfer has been reduced from three
factors, as outlined above, to two factors that is, the resistance of the metal wall and the resistance on
the tank side wall of the coil.
CONCLUSION
Using hot water as the heating medium has some advantages. There is a difference in price between a
steam generating boiler and a hot water boiler, as well as, the requirements for steam pipes, steam
traps and others’ which are not required for water heaters.
In the long run, heating coils/exchanger may need repairs, like sealing small leaks. When a leak occurs
in a steam driven heat exchanger the vacuum created by condensing steam will suck aggressive solution
into the condensate return line and eventually affects the boiler. This danger to the integrity of the
boiler is eliminated by the use of hot water heat. Under these conditions, a leak will flood the tank and
not damage the boiler.
Heat exchangers using steam as the heating medium require less heat transfer area, than water heated
exchangers. Less surface area translates into smaller heat exchangers. These are mostly less expansive.
In addition, steam coils of equal size as water coils, are greatly accelerating the heat up time for the
solution inside a tank.
Copyright © 2014 MBA Manufacturing, Inc. All rights reserved.
MBA Manufacturing, Inc.
1248 Allanson Road, Mundelein, IL 60060
Phone: 847-566-2555 ● Fax: 847-566-2561
Email: mba@mbamfg.com ● www.mbamanufacturing.com