Wrap-Around Enhanced Dehumidification Heat Pipe Heat Exchanger

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Wrap-Around Heat pipe Heat ExchangerFeatures:

  • 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

Areas of Applications: Dedicated Outside Air Facilities With >70%+ Outside Air.  Areas with high-variable latent loads:

  • Hospitals
  • Theaters
  • Fitness Centers
  • Government Facilities
  • Universities
  • Condos/hotels
  • Food & Restaurant Facilities

Optimize Your Energy Efficiency With ACT Heat Pipe Heat Exchangers;

ACT Wrap Around Heat Pipe Heat Exchanger

Large WAHX Wrap-Around Heat Pipe Heat Exchanger

ACT-HP-WAHX systems are used to change the performance of the active cooling coil (chilled water or DX).  In air conditioning and dehumidification applications, especially where the amount of outside air is relatively high (40% or more) for ventilation and indoor air quality purposes, the cooling coil must lower the temperature of the air and also condense out excess moisture.  Reducing the temperature of the air is called sensible cooling and condensing moisture is called latent cooling.

As air passes through the cooling coil, the temperature reduces to the dew point (sensible only cooling).  Once the dew point is met, moisture begins to condense and the temperature continues to reduce (latent and sensible cooling).  In a HP-WAHX installation, the cold air coming off of the cooling coil reduces the temperature of the heat pipes. The cooler heat pipes absorb heat from the incoming warm air stream pre-cooling it prior to reaching the cooling coil.  The sensible cooling performed by the HP-WAHX reduces the initial sensible cooling load on the cooling coil allowing it to more quickly reach the dew point.  The cooling coil can now use more of its capacity to remove latent heat (moisture) and achieve a lower discharge temperature. Essentially, the HP-WAHX is changing the sensible heat ratio of the cooling coil to enhance latent heat or moisture removal.

The lower temperature discharge air holds less absolute moisture (grains of water/pound of air).  Therefore, when it is warmed back up to room temperature, the result will be a lower relative humidity in the conditioned space.  The warm-up process is partially by the free, passive re-heat from the HP-WAHX coil. The design of the ACT-HP-WAHX can be tailored (number of rows, fin pitch, etc.) to achieve the desired amount of enhanced dehumidification.

The U.S. Department of Energy accurately describes the need to not only cool, but also dehumidify your indoor environments to achieve comfort in hot and humid climates.  Air conditioners may not be optimal to achieve both cooling and dehumidification, but the addition of heat pipes to current systems are ideal for hot and humid environments to achieve a comfortable humidity level.  Read more from the US Department of Energy on how heat pipes can help…

Increased Dehumidification Performance and Enhanced AHU Performance Example:

WAHX

 

Everything you need to know about Wrap-Around Heat Exchangers (WAHX)

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.

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