Shell and tube heat exchangers are special type of unit operation which is needed for high-pressure applications; they are durable products, which can withstand the demands of many working environments. Their design plays a large part in ashell and tube exchanger’s ability to endure exceedingly-challenging situations. The gaskets are normally spiral wound gaskets made of stainless steel.

Shell and tube heat exchangers are made from a series of tubes, which can be made of durable material such as fluoropolymers. Fluoropolymers are highly-durable plastics such as PTFE, FEP, and PFA. Fluoropolymers, like heat exchangers, have a place in a variety of industries such as the automotive, medical, and aeronautical.

In a heat exchanger’s shell, one set of tubes contains fluid, which is either heated or cooled. Another set of tubes also contains liquid, which facilitates the heating or cooling of the primary set of tubes. A tube set is referred to as the tube bundle and can take on a variety of shapes depending on what is most conducive for the intended job.

Engineers of shell and tube heat exchangers need to consider several components of construction, among them are:

• A smaller tube diameter enables the shell and tube exchanger to be economical and compact, yet a tiny diameter can facilitate malfunction and difficulty of cleaning. Larger tubing can be instituted to eradicate potential flow and cleaning problems. Engineers must factor cost, space, and the propensity of liquids to foul when constructing a heat exchanger.
• Tube thickness is important to make sure there is room for corrosion; vibration existing in the product has resistance; and, the shell and tube exchanger can withstand pressure coming from both in and outside of its internal tubes.
• Folding or wrinkling the inner tubes increases the flow of the liquid, which facilitates the transfer of heat, producing better performance from the exchanger.
• Designers also consider the layout of the inner tubes. Tubes can be fashioned in a triangular, square, rotated square, or rotated triangular fashion. Particular, internal designs are conducive to specific jobs and the elimination of potential problems such as fouling of the liquid.
• Shell and tube heat exchangers also host baffle components. Baffles serve several purposes such as holding the tube bundles in place; making sure tubes do not sag or vibrate; and, facilitating fluid flow.

Tube Tech International proved that previous industry standards for cleaning such equipment with U bends were inadequate and, for the first time on this unit, Tube Tech gave not only the straight legs, but also the U sections a complete internal clean.

Ever notice how your car seems to run better right after an oil change, especially if you wash and wax it? Well, it’s the same for your furnace… don’t laugh, I’m serious!

The main component of heating units, both forced air and hot water, is the heat exchanger. This component takes the heat produced by burning fuel and transfers it to the water or air for distribution throughout the house. In a hot water system this component is usually concealed from view, and in a forced air unit only 10 to 25% (sometimes it’s completely hidden) of this component is typically visible without disassembly.

Cut-away view of a modern forced-air gas furnaceModern forced-air gas furnace:

1. Solid-state furnace control (Fan assembly visible at lower rear)

2. Draft inducer (fan-forced exhaust)

3. Igniter and flame sensor

4. Gas valve and manifold

5. Gas burners

6. Heat exchanger(s)

7. Air filters

(Configuration will vary between models)

What usually makes heat exchangers inoperative is developing a hole or a crack that allows the hot water to escape, or exhaust from the combustion fuel to escape into the interior air of the home. Constant heating and cooling from years of use will eventually cause a heat exchanger to crack, however some last longer than others. Under ideal conditions, many survive well beyond their predicted life spans.

It seems regular cleaning and maintenance play a factor in life expectancy, as does the environment surrounding the unit. Damp environments tend to assist the build-up of rust on the heat exchanger, shortening its life, while dry, clean environments tend to increase the life span of most furnaces.

Reduced airflow…

Dirty air filters and fan blades, dirty ductwork and obstructed air vents can all contribute to wear on fan motors, reduced efficiency and even premature failure of heat exchangers. Fuel-fired forced-air furnaces are prone to overheating due to obstructions to airflow. Modern furnaces are designed to shut down if temperatures become dangerously high… however, moderately elevated internal temperatures caused by dirt, dust and debris may not be high enough to switch off a furnace, while remaining high enough to cause metal fatigue over extended periods of time.

An annual internal inspection by a licensed burner mechanic or gas fitter, including cleaning and testing for exhaust leaks, should cost between $50 and $100. Considering the implications, I’d say that’s a real bargain! Why not have your furnace inspected, and treat yourself to some peace of mind? For those of you with gas furnaces or wood stoves, a carbon monoxide (CO) detector ($30-$45) is an inexpensive means of protection against the possibility of exhaust leaks, between inspections.

Copyright Gil Strachan – All rights reserved.Gil Strachan is a professional home inspector, representing Electrospec Home Inspection Services in east-central Ontario, Canada since 1994. Visit http://www.allaroundthehouse.com to learn more about home inspections.

“The Home Reference Book”
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