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Heat Wheel Reliability

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There has been much attention paid to all facets of heat wheel performance. An area of concern rarely
addressed is reliability.
A question was posed, “How long should a heat wheel last?” Rather than try to determine a definitive
life span perhaps another question could be posed. “What are the causes of heat wheel failures?”
This paper will take an in depth look at the components of a heat wheel and how each in turn affects
reliability.
A heat wheel is nothing more then a disk mounted on bearings in a framework rotating between two
opposing airstreams.
Components:

Media, Seals, Hub and Bearings, Shaft, Spokes, Motor, Belts, Rims

Segmented rotor or non-segmented rotor:

A design that utilizes a non-segmented rotor by definition requires that the media or heat transfer
substrate bear the physical load of rotation and the lateral stress of the opposing airstreams.

Fig 1. Non-segmented Wheel

Fig 1. Non-segmented Wheel

Fig 2. Segmented Wheel

Fig 2. Segmented Wheel

In Figure 1. Structural loads are
transferred through the media to the
hub.

In Figure 2. Structural loads are
transferred by the spokes to the
hub. The media is not used as a
structural component.

 

 

 

 

The most common failure of a non-segmented wheel is delamination failure at the hub.

Non-segmented Wheel

Non-segmented Wheel

Non-segmented Wheel

Non-segmented Wheel

All of the stress is transferred to
the hub through the media. The
weak link is the glue that binds
the corrugated media at the hub.
The surface area for the
adhesion is smallest near the
hub and this is also where the
stress is largest. It’s clear that
this type of failure is common in
this design.

 

 

 

 

 


Segmented Wheels: All segmented wheels use the same basic components to hold the media in place.
These components are the hub, spokes and rims. The goal is that media is secured and cannot move.

Thermotech Enterprises TF-Series Thermowheel

Thermotech Enterprises TF-Series Thermowheel

Each component must be designed to
handle the loads and stress involved.
The media is easily abraded by any
movement inside the rotor. Although these
movements of the media may be very
small, eventually the media will become
loose and cause a catastrophic failure.

One of the most critical components is the
spoke. Any deflection of the spoke will
allow the media to move and cause
abrasion of the media as detailed in the
pictures below.

 

 

Wheel Damaged by Spoke Deflection

Wheel Damaged by Spoke Deflection

Wheel Damaged by Spoke Deflection

Wheel Damaged by Spoke Deflection

 

The two wheels shown above both failed due to abrasion of the media caused by spoke deflection. The
maintenance staff had tried securing the loose media of the wheel with silicone and expansion foam.
The wheel pictured to the right media had moved enough to cause the wheel to stop rotating. Spoke
deflection should not exceed +/- 1/32” when measured at the outer radius.
There is another advantage to limiting spoke deflection. The less deflection the closer to the rotor the
seals can be set, reducing bypass and increasing performance. There is also no metal fatigue in a
spoke designed to limit deflection.

 


There have been some interesting solutions to wheel deflection issues. This manufacturer employed
a series of casters to limit the rotor deflection of this wheel. Although the outcome of this design did
limit the rotor deflection one can only assume that the casters themselves will become a future
maintenance issue.

Wheel with Casters to Limit Deflection

Wheel with Casters to Limit Deflection

 

 

 

 

 

 

 

 

 

 

There are other problems that can be attributed to media and rotor structure. Some manufacturers
do not use an adhesive to reinforce the corrugated sheets of the heat transfer substrate.

 

Wheel with Deformed Media Due to Lack of Glue

Wheel with Deformed Media Due to Lack of Glue

In this case the weight of the media and
the rotational force of the rotor caused
the media to deform. There was also
substantial media abrasion due to the
movement of the media. This caused
many of the flutes to become closed
greatly reducing the airflow through the
wheel.

 

 

 

 

 

 

 

 


Seals: The seals act as the separation between the opposing airstreams. Properly designed they will
prevent leakage around the rotor. There are two types of seals, contact and non-contact. Contact type
seals are typically made of a soft neoprene or as a brush configuration.

 

Wheel with Brush Seals

Wheel with Brush Seals

Contact brush seals have one disadvantage.
As static pressure across the wheel increases the
seals allow more leakage by the wheel. Brush
seals should only be used in areas where leakage is
not a concern.
Neoprene contact seals tend to wear out causing
increased bypass and another maintenance issue.

 

 

 

 

 

 

Non-contact seals are typically hard rubber and can be as simple as a rubber wiper set as close to
the rotor as possible. Some manufacturers use a labyrinth type seal which uses the differential air
pressure to create vortices that impede airflow pass the seal. This type of seal can be very effective
if the wheel is flat and the rotor does not deflect. One advantage of this type of seal is that as the
static pressure across the wheel increases the labyrinth becomes more effective.

seal1a
What ever type of non-contact seal is used, the important issue
is that the seal does not contact the media. The media face
can easily be damaged by abrasion with the seal.

 

 

The key here is two fold. One, the seal must be

Wheel with Labyrinth Seals Held in Place by Clips

Wheel with Labyrinth Seals Held in Place by Clips

Seal Mounted on an Aluminum Extrusion Bolted to Metal Frame on a Thermotech Wheel.

Seal Mounted on an Aluminum Extrusion Bolted to
Metal Frame on a Thermotech Wheel.

firmly connected to the casing. Two, the wheel
itself must not move. This will be covered later in a
discussion of bearing and hub design.
On the right is a typical non-contact seal held in
place by clips. Although clips can assist in seal
adjustment the seal can also be pulled between
the media and the frame causing severe damage
to the media face.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

In the picture below, the seal has come in contact with the wheel. The damage caused by the abrasion is shown. Note, additional seal clips were added but the media damage is not repairable.

 

Typical Seal Damage Caused by Seal Abrasion

Typical Seal Damage Caused by Seal Abrasion

 

 

 

 

 

 

 

 


Hubs and Bearings: The hub and bearing assembly is crucial to the operation of the heat wheel.
Forces of the wheel in operation are transferred through the hub to the bearings. Failure of either
component will lead to severe damage to the wheel.

Wheel with Media Damage Caused by Bearing Failure

Wheel with Media Damage Caused by Bearing Failure

In this case a bearing setscrew failed allowing the
entire rotor to shift laterally in the casing.
The media was abraded by contact with the seals.
The larger question is, “Why did the setscrew fail?”
This design utilizes a shaft of “off the shelf” round
stock. Round stock has a +0/- 0.004” tolerance.
Bearing specifications call for a + 0.0, – 0.001
tolerance. Over time undersized stock allows for
movement of the shaft in the bearing race.
This leads to setscrew and/or bearing failure.

 

 

 

 

 

 

Wheel with Media Damage Caused by Bearing Failure

Wheel with Media Damage Caused by Bearing Failure

In the picture to the right. The entire rotor has
skewed laterally see figure # 3. This caused
major abrasion of both the seal and the rotor to
the point that there is a 1/4” gouge in the media of
the circumference of the rotor. The seal was
abraded down to the sheet metal and the entire
rotor continued operation to the point that the
sheet metal was holding the rotor in place. This
was caused by a bearing failure.

media4

Wheel with Media Damage Caused by Bearing Failure

 

Here is the opposite side of the same wheel.
Note there is substantial media damage. The
bearing degraded such that the entire rotor could
skew laterally over 1”. As the force of the
airflow pushed the rotor into the seals it was
being abraded on both sides.

 

 

 

 

 

 

Close-up of Failed Bearing

Close-up of Failed Bearing

Figure 3. Showing the Tilt of the Rotor due to Bearing failure

Figure 3. Showing the Tilt of the Rotor due to Bearing failure

 

 

 

 

 

 

 

 

 


Bearings and Shafts: The bearings are key part of any mechanical system. There are two basic
types – bearings that can be lubricated and “Lifetime Lubricated Bearings” which cannot be lubricated.
Bearings that cannot be lubricated are sealed at the factory and can fail due to normal wear and tear or
environmental factors.

Wheel with Permanently Lubricated Bearing in the Return Airstream

Wheel with Permanently Lubricated
Bearing in the Return Airstream

Wheel with Permanently Lubricated Bearing in the Outside Airstream

Wheel with Permanently Lubricated Bearing in
the Outside Airstream

In this case a four year old air handler with an energy recovery wheel had a failure of the bearing in the
outside airstream. This unit was located in central Florida where humidity and temperature can place
extreme loads on the bearing seals. Notice that in the same time period the bearing in the return air was
in considerably better condition. Permanently lubricated bearings have the advantage of requiring no
maintenance. The drawback is rather than adding grease routinely, the only solution is bearing
replacement.

 

 

 

 

 

 

 

There are two types of lubricated bearings used in heat wheels, ball bearings and tapered roller
bearings. Most manufacturers provide ball bearings and, if sized properly and maintained, are a
reasonable solution. On the other hand, tapered roller bearings of the same size can provide for a life
span 10 times that of a simple ball bearing.
Which ever bearing is selected one of the most critical areas that is often overlooked is the shaft that
rides in the bearing. Most manufactures provide a shaft that is “off the shelf” round stock. The diameter
can vary by up to +/- 0.004”. Bearing manufactures suggest +0.0, -0.001 for proper life. If the shaft is
undersized there is a high probability of premature bearing failure.

Thermotech Enterprises TF-Series Unit Tapered Roller Bearings

Thermotech Enterprises TF-Series Unit Tapered Roller Bearings

 

 

 


Motors and Drives: Although drive failures do not typically lead to severe wheel damage they can be an
ongoing source of maintenance issues. Heat wheel manufacturers have tried several different
combinations of pulleys, gears, chains and belts to create a reliable drive system. The goal is basic.
Design the drive system such that the motor can overcome the resistance of the wheel through its entire
operating range while keeping the motor and associated parts as simple and reliable as possible.
Polyurethane belt: This type of belt has the advantage of being both elastic and flexible keeping constant
tension on the drive sheave and rotor. The disadvantage is that after some time, typically 3-5 years, the
material looses its flexibility and becomes brittle. These belts are welded together using a heat source
similar to a soldering gun. The belts tend to fail at this joint unless the weld is expertly done.
Polyurethane interlocked link belts: This type has a better life than the solid polyurethane belt but does
not provide elasticity so the belt must be tensioned by an idler wheel or in some other fashion.
Chain Drive: This type although expensive is reliable. The drawbacks are they require lubrication and
are difficult to repair and replace.
Standard V-belt drive. The standard V-belt has the advantage of superior life, it is easily found at most
supply stores and is easy to repair or replace. This belt is not elastic so some type of tensioner is
required. One manufacture uses a uniquely simple system of gravity tensioned dual V-belts. The motor
is secured to a simple pivot plate so the weight of the motor tensions the belt. This system is extremely
reliable and requires no maintenance.
Motors: The motor should be sized properly for the load and constructed to operate for 10 years
minimum at maximum load. The motor will require some form of speed reduction to turn the wheel at the
required RPM. This is accomplished by the use of reduction sheaves and belts. This adds to potential
failures and increases maintenance needs.

 

Drive with Reduction by Jackshaft Sheave

Drive with Reduction by Jackshaft Sheave

The picture to the left shows one manufacturer’s
drive system. The motor turns a reduction sheave
via a standard V-belt. The reduction sheave is
connected via a jackshaft with two bearings that
transfer the power to the main drive single V-belt
via a spring-loaded idler pulley. There are a
number of failure prone areas and maintenance
issues associated with this complex layout.

 

 

 

 

 

 

Thermotech Drive with Geared Speed Reducer

Thermotech Drive with Geared Speed Reducer

The picture to the right shows a Thermotech drive.
This drive uses a gravity tensioned standard V-belt
and a simple grease lubricated, geared speed reducer.
This design greatly reduces the failure areas and
requires no maintenance.

 

 

 

 

 

 

 

 

 

 

 

 

 


Rims: The rim provides the structure to hold the rotor together and to compress the media, eliminating
movement within the spoke system. Higher compression of the media reduces media movement.

Single Piece Steel Rim Failure

Single Piece Steel Rim Failure

This single piece steel rim broke at the joint with the spoke.
Constant flexing of the connection at the rim bolt connection
caused metal fatigue which eventually caused a catastrophic
failure.

 

Note the size of the screws that join the rim sections.

Note the size of the screws that join the rim sections.

 

 

 

 

 

 

 

 

 

 

 

 

One manufacturer utilizes a welded two piece rim of a unique patented design that allows for individual
compression of each media section. This can provide for a 10X increase in media compression. This
eliminates media movement and the two piece rim design provides enhanced strength eliminating metal
fatigue that caused the single piece rim above to fail.

 


Design Life: A reasonable expectation is an AHU component would last as long as an AHU – 25 years.
The design life of the wheel is determined by the design life of the crucial rotor components. These are
the components that are not easily repaired or replaced. These include the media, hub, shaft and
bearings. The easily replaceable components are the belts, drive and motor. A good starting point for
the lifespan is bearing life. A properly selected tapered roller bearing has been sized for a minimum L-10
lifespan of 290,000 hours, or 25 years. It would then seem a reasonable goal to design all other
critical components to this same standard. Thermowheel® bearings are designed to an L-10 life of
1,000,000 hours so that if poorly maintained they still meet our goal.

 


Reliability Summary:
There are 10 key reliability areas to take into account when selecting a heat recovery wheel.
• Segmented Media
• Low to Zero Deflection Spokes
• High Strength Rims
• Patented rim connection system eliminates media failures
• Tapered Roller Bearings L-10 life 1,000,000 hours
• Shafts Machined To Specified Tolerance
• Simplified Standard V-belt Drive
• Non-contact Low Bypass Seals Bolted To The Frame
• 10 Year Warranty On All Parts And Labor

There is one company that provides all these features and improvements in their heat wheel
products.

Thermotech Enterprises – Zero Failures Since 1985

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