HVAC Energy Recovery

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Passive Energy Recovery for Heating, Cooling and Ventilation Systems

A well-designed passive recovery HVAC heat pipe system will provide effective and affordable energy recovery during the hot summer and cold winter months as well as throughout the year. Advanced Cooling Technologies, Inc. offers a variety of innovative heat pipe HVAC solutions that will take a big bite out of your energy bills and provide a rapid return on your initial investment. We also have the expertise to develop a customized passive energy recovery HVAC heat pipe system for your applications.

Learn more in our HVAC Product Guide

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Highlights of an ACT passive energy recovery HVAC heat pipe system include:

  • Energy cost savings over 40%
  • Quick return on investment from energy savings (1-2 Years Typical)
  • Enhanced dehumidification/latent cooling performance
  • Totally passive, no moving parts or system maintenance
  • Eliminates active overcooling and reheating for dehumidification
  • Engineered efficient & compact design

Wrap-Around Heat Pipe Heat Exchangers

US Department of Energy Cites Heat Pipes as… “Under Utilized” and a viable energy saving technology for HVAC Systems

ACT Wrap-Around Heat Pipe Enhanced Dehumidification Energy Recovery Heat Exchanger: ACT-HP-WAHX

ACT-HP-WAHX Enhanced Passive Dehumidification with Wrap-Around Heat Pipe Heat Exchangers offer engineered performance to enhance your systems efficiency and greatly reduce systems’s energy costs.

  • Quick return on investment (under 2 years) from energy savings
  • Enhanced dehumidification by pre-cooling incoming airstreams
  • Totally passive, no moving parts or system maintenance
  • Installing an ACT-HP-WAHX may result in the choice of a smaller AHU
  • Eliminates typical overcooling to dehumidify, plus free passive reheating
    of the buildings entering airstream

ACT-HP-WAHX Wrap-Around systems can be designed for all major AHU OEMs. For retrofitting existing systems, ACT can ship a pre-engineered unit, fully charged and ready to install. ACT offers onsite installation or units can be factory installed. Typical design build/install costs are recouped in a 1-2 year payback period.

Use our free WAHX Selection Tool to determine the proper WAHX and calculate energy savings estimates.

ACT Heat Pipe Air-to-Air Energy Recovery Heat Exchanger: ACT-HP-AAHX

ACT-HP-AAHX is a counterflow heat exchanger-energy recovery system features ACT’s high performance, high reliability heat pipes. Save energy by pre-cooling or pre-heating your incoming building supply air. The system features:

  • Energy cost savings over 40%, cold or hot climatesair to air heat pipe heat exchanger
  • No cross-contamination between isolated airstreams
  • Economically Improves Indoor Air Quality
  • Quick return on investment from energy savings
  • Reduce Heating or Cooling Requirements
  • Totally passive, no moving parts or system maintenance
  • Engineered efficient & compact design

ACT-HP- AAHX  Heat Pipe Air-to-Air Heat Exchangers can be fitted to new or existing HVAC system. Systems are sealed to prevent cross contamination of the side-by-side airstreams. Qualify for LEED and High  Performance Building points with installation payback periods between 1-2 years.

Use our free AAHX Selection Tool to determine the proper AAHX and calculate energy savings estimates.

Contact Us to Learn More About HVAC Heat Pipes for Passive Energy Recovery

ACT has been providing reliable and affordable thermal management solutions since 2003. Learn more about the many ways in which an HVAC heat pipe system for passive energy recovery can lower your energy bills. Contact ACT for more information or to schedule an in-house consultation today.

Everything you need to know about HVAC Energy Recovery

What functions do air conditioners provide in the summer?

Air conditioners provide two things.  First, they provide space cooling. The cool air delivered to the space quenches the heat being transferred from the outside and heat generating sources in the room.  The air flow rate and the temperature difference between the air delivered to the space and the desired space temperature determine how much heat can be quenched.

The second thing air conditioners provide is dehumidification.  The air conditioner pulls in warm, moist air and lowers the temperature of the air to approximately 52.5°F.  Air at this temperature is capable of holding very little moisture so any excess water vapor will condense as liquid onto the cooling coil.  When that 52.5°F air is warmed back up to the desired temperature for the space, the resulting relative humidity is a comfortable 50%.

What happens if the space temperature is reached before the dehumidification level is achieved?

In many applications, the thermostat set point is reached before the dehumidification level is achieved.  The adage that “more is better” is not true when it comes to air conditioning capacity.  When the thermostat temperature is met and the unit shuts off, then there is no means of dehumidifying the space.  The relative humidity increases.  If the unit’s set point is lowered to address the high relative humidity, then the space becomes over cooled.  Typically, these building spaces have an unfavorable combination of cold temperatures and damp humidity (cold and clammy).

Can I use electrical heat or hot gas reheat to warm the air slightly, so that the unit continues to dehumidify but not over-cool?

Certainly this is one method that can be used; however, using electrical energy to cool the air and then adding more electrical energy to reheat the air will consume a lot of energy. In fact, most building codes and energy conservation regulations prohibit this brute force method.

Hot gas reheat can also be used for more sophisticated refrigerant direct expansion (DX) air conditioners. In these units, there is an extra condenser coil that is placed in the supply air stream.  A fraction of the hot refrigerant gas generated by the compressor bypasses the primary condenser and goes directly to the hot gas reheat coil.  This hot refrigerant gas is used to provide the heating necessary.  While this is an energy efficient method for some DX systems, some systems do not have this capability.

ACT’s wrap around heat pipe heat exchanger (WAHX) can be used to both reduce the cooling tonnage requirement and provide the reheat passively – “for free.”

What types of reheat solutions are available for chilled water coil applications? 

One solution is to include a hot water or steam coil after the chilled water coil as a source of reheat.  Unfortunately, during cooling periods, boilers are often taken out of service because they are no longer required for heat and/or they are too expensive to operate for small loads.

ACT’s wrap around heat pipe heat exchanger (WAHX) can be used to both reduce the cooling tonnage requirement and provide the reheat passively – “for free.”

What is a WAHX?

WAHX is a Wrap-Around Heat Pipe Heat Exchanger.  These are typically used in applications where the air supplied to the space is desired to be warmer than the air temperature coming off of the DX (refrigerant) or Chilled Water coils in an air conditioning unit. By properly designing the WAHX the space cooling requirement and the dehumidification requirement can be met simultaneously without the use of additional DX coils, electrical duct heaters, or hot water/steam coils.

How does a WAHX work?

The WAHX consists of two coils, one before the active cooling coil and one after the active cooling coil.  The two coils are connected by a series of heat pipes, passive two phase heat transfer devices that act like thermal superconductors.  The cold air coming off the active cooling coil reduces the temperature of the heat pipe heat exchanger.  The temperature of the heat pipe heat exchanger will be between the warm air entering the air conditioner and the cold air coming off of the active cooling coil.  Therefore, the first heat pipe coil precools the warm entering air; and, the second heat pipe coil reheats the cold air leaving the active cooling coil.  Typically, the incoming air stream temperature is reduced 5 to 15°F.  The active cooling coil is no longer responsible for this cooling capacity allowing for a smaller capacity active coil.  The energy that is removed from the air stream at the precool coil is returned to the air stream on the down side of the active coil through the heat pipe reheat coil, increasing the temperature by the same 5 to 15°F.

Are there pumps, motors, fans, belts, bearings, compressors, or any other active or moving parts required to make a WAHX work?

Unlike sensible and enthalpy wheels, heat pipes are completely passive.  The heat pipe is fixed in place like any other coil in an air handler and never moves.  Maintenance is essentially zero.  Just keep the filters clean and the heat pipe will operate for twenty (20) years or more.  The heat pipe working fluid, typically a refrigerant like R-134a, evaporates in the precool coil and condenses in the preheat coil.  The liquid refrigerant returns to the precool coil by gravity to repeat the cycle, passively pumping energy around the active cooling coil, reheating – “for free”.

There must be some penalty – nothing is “for free”.

The only penalty on the system is the added fan power to overcome the additional pressure drop for the two WAHX coils.  These two coils are typically 2 to 4 rows deep and have fin pitches in the eight (8) to fourteen (14) fins per inch range.  At a common face velocity of 450 FPM, this translates to an additional pressure drop of 0.27 to 0.71 inches of water (IWG).  The most common WAHXs average 0.50 IWG pressure drop.  The additional pressure drop reduces the WAHX efficiency by 10%.  In other words, the energy consumption of the fan increases by less than 10%, but the WAHX provides more than 10% in cooling energy savings – this is a net win!

What does WAHX effectiveness mean?

The effectiveness of a WAHX is the amount of energy transferred by the heat exchanger relative to the maximum possible amount of energy that could be transferred for the conditions that it is exposed.  For sensible only devices like heat pipe heat exchangers, the effectiveness is typically reported as temperature effectiveness.  For a typical 100% Dedicated Outdoor Air System (DOAS), the outdoor air may be entering at 90°F and the leaving air temperature off the cooling coil is 50°F.   The maximum temperature difference is 90-50 = 40°F.  In a WAHX application, the flow rate through the precool coil and reheat coil are equal.  Therefore, 40°F is the maximum possible temperature difference that could be achieved.  If the 90°F stream is cooled to 80°F, then the temperature effectiveness of the WAHX is (90-80)/(90-50) = (10/40)= 25% effectiveness.

Is a higher effectiveness WAHX better than a lower effectiveness WAHX?

While effectiveness is a quick way to determine expected temperatures through a system at various conditions, for WAHXs the adage that “more is better” is not true.  Architectural and engineering firms determine the air conditioning requirements for a space depending on its intended usage, heat sources, heat leakage, etc.  Based on their calculations, they may set the specification for a 100% Direct Outside Air System (DOAS) at 10,000 CFM, cooled to 55°DBF/54°WBF, and reheated to 65°F before being delivered to the space.  The maximum outside air temperature for that location is 95°F.

The temperature effectiveness desired is (65-55)/(95-55) = (10/40)= 25%.  Providing a WAHX with a higher temperature effectiveness will cause the reheat temperature to be too high.  For example if the WAHX effectiveness was 40%; then the reheat temperature would be 71°F [40% = (X-55)/(95-55)].  The bottom line is that WAHX should be precisely designed and built to a target effectiveness or should be controllable to adjust to changing conditions.

How can a WAHX be controlled?

The basic concept for WAHX control is to design and build the highest expected effectiveness required WAHX and partially disable functionality to adjust the effectiveness to the current conditions.  This may mean selecting a six (6) row WAHX and having a method of disabling several of the rows.  Typically, the Building Management System (BMS) for the air handler is programmed to add the heat pipe control to the operating sequence.

Often the lowest cost method of control is to bypass some of the air stream around the WAHX reheat coil.  The amount of bypass can be used to limit the reheat temperature.  The other common control techniques require valves in the heat pipe circuit, either individual pipe valves or one valve per row for highly circuited systems, like split loop thermosyphon type designs.

What is a Split Loop Thermosyphon WAHX?

In a split loop thermosyphon (SLT) type design, precool and reheat coils are rotated 90°, such that the coil tubes are vertical.  Each row of vertical tubes is brazed into a top and bottom manifold.  The top manifold is a vapor tube that will transfer the evaporated vapor from the precool coil to the reheat coil.  The bottom manifold is a liquid tube that will return condensed liquid from the reheat coil to the precool coil.  The reheat coil is placed slightly above the precool coil such that liquid can be returned by gravity. As the liquid boils in the precool coil, the vapor generated naturally rises and travels toward the reheat coil to condense and complete the loop.

Control is simple and cost effective by placing a refrigerant grade, actuated ball valve in the vapor line between the precool and reheat coils.  Closing the valves stops the flow of vapor, essentially shutting down the SLT. Valves can be added to all or just some of the coil circuits to shut down various proportions of the unit for more complete control.

Are there any limits to the design of a Split Loop Thermosyphon WAHX?

Split loop thermosyphons rely on the boiling of the refrigerant in the vertical tubes to keep the entire length of the tube wet with fluid, ready for evaporation.  If the tubes are too long, gravity prevents the boiling action from splashing liquid to the higher region of the tubes.  These regions are no longer effective for evaporative heat transfer.  Therefore, SLT are limited to about 36” of fins in the vertical direction.  If more than that is required, the units are stacked.

Are there any online tools to help design a WAHX?

ACT’s online WAHX selection tool / calculator can be used to design and evaluate the performance of a WAHX subjected to various conditions.  It can be found at www.1-ACT.com/HVAC/WAHX.  No login or account setup is required.  Simply enter the dimensions of the coil (or the face velocity), flow rate, entering air conditions, leaving active cooling coil conditions, and desired reheat temperature.  Press the calculate button.  The results are available as a .pdf or you can submit the result to ACT for review.  You can also put the calculator into “rate mode” and fix the number of rows and the fins per inch to see how a particular design will operate under various conditions.

How is indoor air quality maintained in buildings?

One of the most common techniques to manage indoor air quality is to introduce a certain amount of outside air to dilute the inside air.  You may have heard the term “Air Turns per Hour”.  If the air turns per hour in a space is ten, that means that every hour ten times the volume of the space is brought in from outside.  The same amount of indoor air is expelled through dedicated ventilation ducts, leakage through doors, windows, bathroom vents, etc.

Maintaining a favorable indoor air quality through managed ventilation sounds expensive, like leaving the windows open in the winter? 

As energy conservation has become more and more important, for environmental and cost reasons, recovering energy from ventilation air streams has become the norm.  In fact, many building codes dictate when and how much energy recovery is needed in various building applications.  By exchanging energy between the leaving ventilation stream and the entering outside air stream, the impact of ventilation can be minimized; and simultaneously, indoor air quality can be excellent.

What type of heat exchangers can be used to recover the energy between the outdoor air and ventilation exhaust?

There are three main types of heat exchangers; energy wheels, enthalpy plates, and heat pipes.  All are effective devices at exchanging energy between two air streams.

How does an energy wheel work?

When the ventilation duct and the outside air duct are side by side a wheel type heat exchanger can be used.  Half of the wheel is in one duct and half of the wheel is in the other. As the wheel spins, the portion of the wheel that is in the warmer duct heats up the porous wheel structure.  The portion of the wheel that is in the cooler duct removes heat energy from porous wheel structure.  Wheels can also be treated with a desiccant such that moisture can also be transferred from one stream to the other.

Wheels do not seal perfectly and the volume of the wheel itself leaks from one duct to the other.  So, applications requiring zero transfer (cross contamination) between streams cannot use wheels.  Wheels require side by side and equal sized ducts, limiting the packaging freedom that many applications require.  And last but not least, wheels are active moving machines with motors, belts, pulleys, bearings, etc. requiring highly skilled maintenance personnel to keep the wheels turning.

How does an enthalpy plate exchanger work?

Plate exchangers are a series of parallel plates stacked together with each passage sealed from the next such that one airstream passes through every other passage on one face and the other airstream passes through every other passage on a face 180° apart.  The warmer air stream in one channel conducts heat through the plate material into the cooler air stream.  Some plate exchangers have desiccant membranes so that thermal energy and moisture can pass from one stream to the other.

Enthalpy plates can be sealed fairly well such that cross contamination is negligible.  Like wheels, they require side by side duct layouts. So, packaging limitations prevent plates from being used in many applications. Besides the limited packaging freedom of having to have the ducts side by side and equal size, the packaging freedom is further constrained by their large size and volume.  A typical plate core can be 36” x 36” and placed on a diagonal in an air handler.  That requires about 51” of air handler length to accommodate!

How does a heat pipe heat exchanger work?

Heat pipes are thermal superconductors that use the evaporation and condensation of a refrigerant inside of a closed tube to transfer heat from one end of the closed tube to the other.  As heat is applied to one end of a heat pipe, the liquid refrigerant inside begins to boil converting liquid refrigerant to vapor refrigerant.  The slightly warmer vapor refrigerant is at a higher pressure than the vapor refrigerant at the other end of the heat pipe.  This pressure difference causes the warmer vapor to move to the lower pressure, cooler end of the pipe.  Here the vapor condenses on the cooler surface giving up the latent heat of vaporization of the refrigerant as it converts back to a liquid.  The liquid returns to the warmer end by gravity flow for another round of evaporation and condensation.  In a properly sized heat pipe, this process can transfer large quantities of thermal energy (heat) over long distances with practically no temperature difference, typically one or two degrees Fahrenheit,  from end to end.  Therefore, this heat pipe element, or superconductor of heat, can be used to build a superconducting heat exchanger.

Heat pipe heat exchangers start out as conventional tube and fin coils, where each of the tubes is converted into a super conducting heat pipe element.  The warmer air stream passes over one portion of the coil and the cooler air stream passes over the remaining portion of the coil, usually in a counter flow arrangement.  The warm air evaporates the refrigerant on the one side and the cool air condenses the refrigerant on the other side transferring large amounts of heat from one air stream to the other.

Heat pipe heat exchangers can be any size that a conventional cooling coil can be built.  They can be long and short or short and tall.  The two air stream sizes (ducts) can be different size and shape. The two heat pipe coils can also be separated by tens of feet.   They can also be designed to have 0% cross contamination and they only require a small section of air handler length to install – typically less than 12”.

What is an AAHX? 

AAHX is an Air-to-Air Heat Pipe Heat Exchanger.  These are typically used in applications where there is a ducted exhaust stream.  Air that is exhausted from a building, to maintain favorable indoor air quality, has been cooled (summer) or heated (winter) to the typical space temperature of 70 to 75°F.  This exhaust stream and an AAHX can be used to precool or preheat the incoming, outside make-up air.  In the summer, when the temperature outside is above the 75°F exhaust stream temperature, the AAHX will precool the outside air. In the winter, when the temperature outside is less than the 70°F exhaust stream temperature, the AAHX will preheat the outside air by exchanging energy in the AAHX.

What does AAHX effectiveness mean?

The effectiveness of an air-to-air heat pipe heat exchanger is the ratio of the energy transferred versus the maximum possible amount of energy that could be transferred for the conditions that it is exposed.  For sensible only devices like heat pipe heat exchangers, the effectiveness is typically reported as temperature effectiveness.  For a typical occupied building application, the outdoor air, required for ventilation, may be entering at 90°F and the temperature of the exhaust air is 75°F.   The maximum temperature difference is 90-75 = 15°F.

In an AAHX application, the flow rate through the precool coil and reheat coil are typically nearly equal, although they do not have to be.  In fact, most building ventilation application will have more outside air versus exhaust air because some air is lost through non-ducted exhaust, like doors and windows, bathroom fans, etc.  The effectiveness of the streams are corrected for the dissimilar flow rates by a ratio of the flow rate of the side being evaluated divided by the smaller of the two flow rates.

Let’s look at an example.  Outside air at 90°F is being pulled into the building at 10,000 CFM and is being precooled to 80°F by the AAHX.  Exhaust air at 75°F is being vented from the building at 8000CFM.  Therefore, 90-75=15°F is the maximum possible temperature difference that could be achieved.  The actual temperature difference of the precooled air is 90-80=10°F.  So, the temperature effectiveness of the outside air stream is (10,000/8,000) x (90-80)/(90-75) = 83.3%.   The exhaust air temperature will be 83.3% = (8,000/8,000) x (X-75)/(90-75) or X=87.5°F.

Is a higher effectiveness AAHX better than a lower effectiveness AAHX?

For applications trying to achieve maximize energy recovery, the adage that “more is better” is true.  For example, in the most common application of AAHXs, energy recovery from of the building ventilation air stream, maximum recovery is best.  Therefore, AAHXs are typically 6, 8 or 10 rows deep of heat pipes.  The choice of rows depends on cost considerations and the ability to overcome the additional pressure drop on the system.

How can an AAHX be controlled?

AAHXs, like all heat pipe heat exchangers rely on gravity to return the refrigerant liquid from the condenser (cool) end of the heat pipe heat exchanger to the evaporator (warm) end.  In a horizontal application where the ducts are side by side, if the AAHX is perfectly level, it will work in both directions; however, if the heat pipe is out of level by even the smallest amount the liquid will only flow by gravity in one direction rendering it useful in only one season.

To address this and to achieve a method of control, a tilting mechanism is often supplied with the AAHX.  The AAHX casing at the bottom center is designed with a hole that accepts a nominally 1” diameter shaft.  The shaft is mounted onto a strong base plate made of steel channel through a pillow block bearing.  A secondary, stationary casing is built off of the base plate approximately 4 to 6 inches larger than the coil casing.  Flexible duct connectors are used to connect the tilting coil to the stationary frame, maintaining the zero cross contamination seal.  An electric actuator is used to tilt the AAHX approximately one inch up and down.  In summer, it is tilted such that the outside air stream is low and in the winter it is tilted so that the exhaust stream is low.  Modulating the tilt can be used anytime to disable the heat exchanger.  Often this feature is used to defrost a coil that has frozen in very cold conditions.

What is a Split Loop Thermosyphon AAHX?

In a split loop thermosyphon (SLT) type design, precool and reheat coils are not one continuous flat slab with a center divider.  In fact the two coil sections may be separated by many feet and/or oriented at an angle to each other.  In a SLT AAHX, the ½” diameter coil tubes are rotated 90°, such that the coil tubes are vertical.  Each row of vertical tubes is brazed into a top and bottom manifold.  The top manifold is a vapor tube that will transfer the evaporated vapor from the precool coil to the reheat coil.  The bottom manifold is a liquid tube that will return condensed liquid from the reheat coil to the precool coil.  The reheat coil is placed slightly above the precool coil such that liquid can be returned by gravity. As the liquid boils in the precool coil, the vapor generated naturally rises and travels toward the reheat coil to condense and complete the loop.

Control is simple and cost effective by placing a refrigerant grade, actuated ball valve in the vapor line between the precool and reheat coils.  Closing the valves stops the flow of vapor, essentially shutting down the SLT. Valves can be added to all or just some of the coil circuits to shut down various proportions of the unit for more complete control.

Are there any limits to the design of a Split Loop Thermosyphon AAHX?

Split loop thermosyphons rely on the boiling of the refrigerant in the vertical tubes to keep the entire length of the tube wet with fluid, ready for evaporation.  If the tubes are too long, gravity prevents the boiling action from splashing liquid to the higher region of the tubes.  These regions are no longer effective for evaporative heat transfer.  Therefore, SLT are limited to about 36” of fins in the vertical direction.  If more than that is required, the units are stacked.

Are there any online selection tools to help design a AAHX?

ACT’s online AAHX selection tool / calculator can be used to design and evaluate the performance of an AAHX subjected to various conditions.  It can be found at www.1-ACT.com/HVAC/AAHX.  No login or account setup is required.  Simply enter the dimensions of the coil (finned height and finned length), flow rates, entering air conditions, and one desired leaving air temperature.  Press the calculate bottom.  The results are available as a .pdf or you can submit the result to ACT for review.  You can also put the calculator into “rate mode” and fix the number of rows and the fins per inch to see how a particular design will operate under various conditions.

 

 

 

 

 

 

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