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Heat Pipe Technology split passive (HRM-V™) energy recovery heat pipes are an air to air heat exchange system designed to move heat from one air stream to another. Split passive HRM-V™ heat pipes provide economical and reliable recovery for summer only, winter only, or all season applications where supply and exhaust air streams are remotely located. These systems are designed for both process and comfort applications to pre-cool or pre-heat outside air using otherwise wasted heat from exhaust air. Natural thermosiphon is used to circulate refrigerant, from the supply air coil to the exhaust air coil where it changes phase from a liquid to a gas and back to a liquid. Because of this natural phase change unique to heat pipes, the HRM-V™ system can produce significantly higher heat transfer capabilities than a comparable water glycol system often yielding 40% or more. Best of all, because they have no moving parts HPT heat pipes require minimal maintenance and provide passive, reliable energy recovery.


HRM-V-Construction Figure 1 - Typical dimensions

Coil construction consists of tube and fin design with copper tubes and aluminum or copper fins. The coils are encased on top, bottom and sides with either galvanized or stainless steel casing for easy installation into air handling units or duct work. Headers are type L copper. Header size will vary based on the capacity of the coil. Each coil is divided into multiple circuits, to be piped in a counter flow arrangement, and individually charged for maximum heat transfer effectiveness.  The tubes of the coil are configured vertically with fins oriented horizontally. The rows of tubes for each circuit are manifolded to a vapor header at the top of each circuit and a liquid header at the bottom of each circuit. Systems can be designed with two rows per header or one row per header depending on the performance requirements of the application. The HRM-V™ system can be configured with 2, 4, or 6 rows. However, a typical HRM-V™ heat pipe system will use 6-rows. Air pressure drop and fan power increase with each row. Coil depth is based on the number of rows.



Figure 2 - Typical Dimensions

Installation Configurations

When installed on the same elevation, the HRM-V™ system can transfer heat in both directions making it ideal for summer and winter recovery. For installations where air stream is elevated, the HRM-V system can only transfer heat from the lower air stream to the higher air stream, therefore, limiting heat transfer to one season. HPT offers the following arrangements;


Winter Only Recovery

Figure 3
  1. Winter Only Recovery
  2. Summer Only Recovery
  3. Winter/Summer Recovery

A. Winter Only Recovery

When cooler outside air is above warmer exhaust air

-Maximum 100* ft equivalent horizontal separation

-Maximum 50 ft* equivalent vertical separation

*Valves and elbows shorten this distance

Summer Only Recovery

Figure 4


B. Summer Only Recovery

When warmer outside air is below cooler exhaust air

-Maximum 100 ft* equivalent horizontal separation

-Maximum 50 ft* equivalent vertical separation


*Valves and elbows shorten this distance


Winter/Summer Recovery

Figure 5

C. Winter/Summer Recovery

When outside air is on the same elevation as exhaust air

-Maximum 40 ft equivalent horizontal separation


*Valves and elbows shorten this distance





Comfort Energy Recovery

HPT split passive energy recovery heat pipes can be used for comfort-to-comfort applications or for process applications. Comfort-to-comfort applications include heating only recovery for cooler climates, cooling only recovery for warm climates, or more often, both heating and cooling recovery.  Split passive energy recovery heat pipe systems are being used for heating/cooling recovery from cold northern zones with harsh winters to the heat of more southern climates.

Process Energy Recovery

For process applications, heating/cooling recovery can also take place in either direction.  Process applications frequently involve air temperatures elevated above normal room conditions.  The heat pipes can be made to withstand temperatures up to 200°F.  For air streams with corrosive components, the heat pipes can be provided with a protective coating. Heat pipes can also be fabricated with both fins and tubes made of copper.

HRM-V- Counter flow piping

Figure 6 - Counter flow piping


Design Considerations

The HRM-V system allows for multiple coil sections to be utilized for applications with larger airflows. The HRM-V system design permits one supply coil to be used for each exhaust coil. Furthermore, supply and exhaust coils can be sized differently to handle unequal supply and exhaust airflows. Above in Figures 3-5 the different split passive energy recovery heat pipe orientations are shown. For maximum heat transfer, air streams must be piped in counter flow. Counter flow operation is where two air streams are arranged such that they flow in opposite directions through the heat pipe coils. If necessary, due to design considerations, the air streams may need to flow in the same direction through the heat pipe. In this case counter flow can still be achieved through connecting the liquid and vapor headers in a counter flow fashion. See Figure 6.



For an HRM-V system where coils are located on the same level, the system will transfer heat from whichever end is warmest in either direction. For a HRM-V system where the warmer airstream is lower, the system will transfer heat in one direction. Heat transfer is due to the entering air temperature difference and the latent heat of evaporization and condensation capabilities of the working fluid within the system. With a small difference in temperatures from one side to the other, a percentage of that difference is transferred to the other end. As the temperature difference increases, the difference in temperature across each end increases. Operation of heat pipes is automatic by refrigerant circulation and needs no further attention. The coils are fully pressure tested and will give many years of trouble-free operation.