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.”

WAHX is a Wrap-Around Heat Pipe Heat Exchanger. These are typically used in applications where the desired air supplied to the space is 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.

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 by 5°F 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 airstream at the precool coil is returned to the air stream on the downside of the active coil through the heat pipe reheat coil, increasing the temperature by the same degree of difference ~ 5°F to 15°F.

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…

Unlike sensible and enthalpy wheels or energy 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. By keeping the filters clean, the heat pipes 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”.

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!

The WAHX should be used around a chilled water coil. It can be in the same duct or tunnel, but the WAHX should be installed such that hot water coil is not in between the two wrap-around coils.

When used with a chilled water coil, there will be condensation on the pre-cool coil under some use conditions, necessitating a drain pan in the design. The reheat coil should be installed at least 6in after the chilled water coil to allow the condensation to drop off and not land on the reheat coil. When installed in this way, there should be no condensation on the reheat coil or on the outside of the pre-cool coil. We recommend that the pre-cool coil is installed directly on the chilled water coil which allows all condensation that forms on both coils to be collected by a smaller drain pan installed underneath these coils.

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.

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 target effectiveness or should be controllable to adjust to changing conditions.

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 such as a split loop thermosyphon design.

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.

ACT HVAC Split loop thermosyphon systems (SLTS) rely on the boiling of the refrigerant in heat transfer coils that are headered vertically. The boiling occurs in the hotter airstream which keeps the entire length of the vertical tubes wet with fluid, ready for evaporation and heat transfer. If the SLTS 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. As the refrigerate boils, the vapor that is created is transferred via upper vapor lines to the cooler condensing coil. From the condensing coil lower-level, liquid lines return the condensed vapor liquid state refrigerant to be once again boiled in the hotter airstream and vaporized to create the thermosyphon heat transfer between the hot and cold coils. Therefore, ACT SLTS are limited to 37. 5” of vertical fin height to maintain proper heat transfer. ACT WAHX or AAHX systems can be stacked to create fin heights greater than 37.5”.

Yes! ACT’s online WAHX selection tool can be used to design and evaluate the performance of a WAHX subjected to various conditions.  It can be found at our resouces page.  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 change the calculator to “rate mode” and fix the number of rows and the fins per inch to see how a particular design will operate under various conditions.

The ACT-WAHX system is made up of two coils that are interconnected pipe-to-pipe. Each interconnected pipe is an individual heat pipe. There is a precool heat pipe coil and a reheat heat pipe coil. The AHU’s chilled water or DX coil is installed between the two WAHX coils.

It is typically recommended that a drain pan be installed under the full area of all three coils. At the very least a drain pan should accommodate the WAHX precool coil and chilled water or DX coils. All effort should be made to block any air bypass on the top, sides and bottom of the WAHX’s precool or reheat coils. The chilled water or DX coil should be properly cased as well to avoid air bypass.

The GAP distance between the WAHX precool and reheat coils is up to the engineering designer. Most designers try to allow the space between the three coils to accommodate cleaning or space to install temperature or pressure sensors.  Some applications need the space for UV decontamination lighting.

The minimum installation for a WAHX system would be the WAHX precool coil sitting flush with the cooling coil and the WAHX reheat coil at least 6.0” or more distance from the cooling coil.  This avoids any possibility of water from the cooling coil being blown onto the WAHX reheat coil which could cause evaporative cooling.  It’s always best if room allows for a 24.0” gap between the coils.

ACT offers stainless steel casing and optional Electofin epoxy corrosion fin coating or Heresite corrosion coating. These options are used when the external environment has chemical concerns like hydrogen sulfide in chemical process cooling applications or air handlers installed in relative proximity (up to several miles) to saltwater coastal areas. These options provide for a long installation life of up to 15+years.