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.

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 applications will have more outside air versus exhaust air because some air is lost through the 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.

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

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.

In an AAHX Passive-Split Loop Thermosyphon (SLT) Heat Exchanger systems pre-cool and re-heat 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 Split Loop Thermosyphon 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 Split Loop Thermosyphon. 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”.

Using our free online selection tool, you can design and evaluate the performance of an AAHX subjected to various conditions.

HVAC design selection and evaluate performance with our free online selection tool

1. Simply enter the dimensions of the coil (finned height and finned length)
2. Flow rates
3. Entering air, conditions
4. Enter desired exiting air temperature
5. Press the calculate button- The results are available as a .pdf or you can submit the result to our engineers for review
6. Try “rate mode” and fix the number of rows and the fins per inch to see how a particular design will operate under various conditions