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The Dawn of the 10 Year Labor Warranty

Energy Exchange Rotary energy recovery wheels operate by exchanging sensible and latent energy from two opposing airstreams. Known as a heat wheel, it is a simple mechanical device that if designed and constructed properly can afford a high degree of energy recovery, often up to 90 percent. Heat recovery wheels are extremely efficient because they transfer total energy (heat and moisture) between two airstreams.

I haven’t been shy making my feelings about energy recovery wheels known. I understand their benefits, but in my opinion these have always been overshadowed by their glaring faults: excessive maintenance requirements, cross contamination, short warranties, and shoddy construction. An energy recovery wheel that effectively overcomes all of these problems in our humid environment would be fantastic, because it would do its job (primarily shaving down coil requirements for outside air and high latent processing) while running trouble free.

Enter Thermotech Enterprises. Their Thermowheel Energy Recovery Wheels are built like tanks and feature a standard 10 year parts and labor warranty (yes, that is not a misprint). Their Energy Recovery Wheels are designed for new air handler applications in the 500 to 150,000 CFM range and are easily integrated into JCI/YORK cabinets or assembled in the field.

Thermotech Enterprises also has a series of Energy Recovery Wheels that are a direct field replacement for existing heat recovery wheels manufactured by Berner, Cargocair, SEMCO, Regenerative Heat Corp., Enreco, Governair, Flakt, Wing, Carnes, Heat Recovery Corp, and Munters.

There has been a lot of attention paid to all aspects of heat wheel performance. An area of true concern that is rarely addressed is reliability. A question I hear a lot is “How long should a heat wheel last?” Rather than try to determine a definitive (and possibly arbitrary) life span perhaps we should instead ask the question, “What are the causes of heat wheel failures?” In this article we will take an in-depth look at the components of a heat wheel (media, seals, hub and bearings, shaft, spokes, motor, belt, and rims) and see how each in turn affects the overall system’s reliability.

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. 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. In the case of Figure 2 the media is not used as a structural component.


Figures 1 & 2: Segmented and Non-segmented Wheels

The most common failure that we see in a non-segmented wheel is delamination failure at the hub. All of the stress that the wheel incurs is transferred to the hub through the media. The weak link is the glue that binds the corrugated media at the hub (see Figure 3). 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 a byproduct of the design.


Figure 3: Non-segmented Wheel Delamination

Figure 3: Non-segmented Wheel Delamination

 

Segmented Wheels

All segmented wheels use the same basic components to hold the media in place. These components are the hubs, spokes, and rims (Figure 4). Each component must be designed to handle the loads and stress involved. The goal is that media is secured and cannot move because it is easily abraded by any movement inside the rotor. Although these movements of the media ma be very small, eventually the media will become loose, potentially causing 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 Figures 5 & 6.


Figure 4: Thermotech TF-Series Wheel Components



Figures 5 & 6: Wheel Damage as Result of Spoke Deflection

The two wheels in Figures 5 & 6 both failed due to abrasion of the media caused by spoke deflection. The maintenance staff had attempted to secure the loose media of the wheel with silicon and expansion foam. The media in Figure 6 had moved so much that the wheel was no longer able to rotate. As a reference, spoke deflection should not exceed +/- 1/32″ when measured at the outside 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. The manufacturer in Figure 7 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.


Figure 7: Wheels with Casters to Limit Deflection

There are other problems that can be attributed to media and rotor structure. Some manufacturers, such as the one in Figure 8, do not use an adhesive to reinforce the corrugated sheets of the heat transfer substrate.


Figure 8: 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.


Figures 9 and 10: Brush Seals versus Neoprene Seals

Contact brush seals have one disadvantage. As static pressure across the wheel increases the seals allow more leakage by the wheel. Brush seals (Figure 9) 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. Thermotech uses a labyrinth type seal which uses the differential air pressure to create vortices that impede airflow pass the seal (Figure 10). 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.

Whatever 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 (Figure 11).


Figure 11: Typical Seal Damage Caused by Seal Abrasion

The key here is two fold. One, the seal must be 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. Below 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 Figure 12 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.


Figure 12: Wheel with Labyrinth Seals Held in Place by Clips

 

Hubs & Bearing Assembly

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. In the case of Figure 13 a bearing setscrew failed allowing the entire rotor to shift laterally in the casing.


Figure 13: Wheel with Media Damage Caused by Bearing Failure

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.

In Figure 14 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.


Figure 14: Another Wheel with Media Damage Caused by Bearing Failure

 

Bearings & 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.


Figure 15: Wheel with Permanently Lubricated Bearing in Return Airstream.
Figure 16: 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 (Figure 16). 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 (Figure 15). 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 (Figure 17). 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.


Figure 17: Thermotech TF-Series Tapered Roller Bearings

 

Motors, Belts, & 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. Thermotech 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.

Figure 18 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.


Figure 18: Drive with Reduction by Jackshaft Sheave

Figure 19 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.


Figure 19: Thermotech Drive with Geared Speed Reducer

 

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. The single piece steel rim in Figure 20 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.


Figure 20: Single Piece Steel Rim Failure

Thermotech 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 that 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 219,000 hours, or 25 years. It would then seem a reasonable goal to design all other critical components to this same standard.

 

Reliability  Summary

There are 8 key reliability areas to take into account when selecting a heat recovery wheel.

  • Segmented Media
  • Low to Zero Deflection Spokes
  • High Strength Rims
  • Tapered Roller Bearings
  • Shafts Machined To Specified Tolerance
  • Simplified Standard V-belt Drive
  • Non-contact Low Bypass Seals Bolted To The Frame
  • Minimum 10 Year Warranty On All Parts And Labor

Thermotech Enterprises has had zero heat wheel failures since 1985 and is the only company that provides all of these features and improvements in their heat wheel products. McNellage & Associates can help you with all aspects of heat wheel selection and air handler design. Every year we work with owners, architects, engineers, and contractors all over the Gulf Coast to design energy-saving HVAC equipment. We would love to work with you and your team! Click here to contact us and learn how to make your HVAC systems starting saving you thousands of dollars a year in energy costs.

 

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