Last Fall, a building manager stopped at ACT’s booth at the WEEC tradeshow and explained that he was getting ready to contract a design for an HVAC system and that new environmental regulations required energy recovery to be considered in the design. He was hoping that we could provide him information that would enable him to make this an appealing addition to the system when he presented the plans to the building’s owners. For his particular situation, we recommended looking at a Wrap Around Heat Exchanger with control options for the biggest cost savings.
We realized this might be a common event, and therefore are providing the following information as an explanation to all of our customers trying to build an economic case for an energy recovery system. There is a large impact to adding controls to an energy recovery system. It has the potential to not only reduce the size of the AHU (air handler), but also eliminate the need for a reheat system.
A dedicated outside AHU using chilled water can be greatly enhanced by the addition of a Passive Wrap-Around Heat Pipe Heat Exchanger (WAHX). A properly engineered WAHX system saves thousands of dollars per year in energy cost with payback periods in under two years.
The WAHX performs two major functions:
- It reduces the outside air temperature through the use of a WAHX pre-cool coil prior to the chilled water coil. Not only can this be equated to tonnage savings, it also provides enhanced dehumidification, as the outside air humidity readily condenses on the WAHX pre-cool coil.
- After the chilled water coil, the WAHX re-heat coil proportionally applies the heat taken out by the pre-cool coil to the air leaving the AHU. This results in dry neutral air being emitted to the facility. The WAHX performs this function passively with no external pumps or motors.
Table 1 (Temperatures are in Fahrenheit)
In Table 1, we show the behavior of the 4 Row WAHX in a 20,000CFM Air Handler Unit (AHU). At 92°, the pre-cool coil removes 15.2° and adds 15.1° of re-heat. These delta temperatures are always in relative balance.
Every AHU system is configured for the design point temperature for the region. A WAHX system utilizes this data to select the number of heat pipes that are contained in the WAHX to deliver the desired re-heat temperature at design point. In Table 1 above, the design point is 92°DB/77.7°WB . Our cooling coil is 52.5°DB/52.4°WB and the desired re-heat is 67.6°DB/58.5°WB. Placing these three variables, our CFM, and coil size into the ACT WAHX Selection Tool yields a result of a 4 Row WAHX to deliver the desired 67.6°DB/58.5°WB reheat temperature.
As you can see in Table 1, as the outside temperature increases, the reheat temperature increases. As the outside temperature falls, the reheat temperature falls. This is called “part load” performance. In some applications having the WAHX re-heat temperature vary is a desired function of the WAHX. If the engineer wants to maintain a specific level of reheat even at part load more heat pipes (rows) must be added. Remember whatever heat is removed by the pre-cool coil is added by to the WAHX re-heat coil.
Adding Control for Part Load Reheat Performance
ACT features three distinct ways that a WAHX system can maintain a desired level of reheat under part load conditions. To control reheat in a WAHX system we must either add (turn on) heat pipes for more reheat or (turn off) heat pipes for less reheat. So as the outside temperature increases we would turn off the WAHX’s individual heat pipes, or the rows of heat pipes, so as not to over shoot the desired reheat target and make the building too warm. The opposite function of turning on rows is done as the outside temperature decreases.
We’re in Control
As mentioned ACT has three distinct ways to maintain part load desired reheat performance. The first method of control is with the patented ACT-TPV Thermal Passive Valve (ACT-TPV). This on/off device is set at a factory selected on/off temperature and automatically turns the WAHX heat pipes on & off based on the outside temperature. Each TPV has a paraffin plunger switch that melts at a specific temperature to turn off the heat pipe. As the outside rises the paraffin melts and turns off the heat pipe. As the outside temperature drops the paraffin solidifies and turns the heat pipe on again. The system requires no outside control to maintain the reheat desired reheat temperature. There is one ACT-TPV per individual heat pipe.
The next level of controls is the ACT-Active Thermal Valve (ACT-ATV). We once again need one valve switch per individual heat pipe. This active thermal switch uses the same paraffin plunger to turn the heat pipe on/off by a small embedded heater that is controlled by the buildings management systems. The embedded heater melts the paraffin to close or turn off the heat pipe to reduce reheating. If the power fails to the control system all of the ACT-ATV fail in the open position to allow the heat pipe to continue to operate.
The final WAHX reheat control system is featured with a WAHX split loop coil configured system. This system does not use individual heat pipe tubes to transfer energy but utilizes a split loop thermosyphon principle. The WAHX Split Loop Thermosyphon System has a vapor line and a liquid line. As the hot outside air encounters the pre-cool coil the refrigerant inside of the system starts to boil. The internal temperature differential causes a pressure differential between the pre-cool coil and reheat coil which starts the thermosyphon, which transfers the heat. A valve is placed in series with each row’s vapor line and can be controlled to turn the entire row on and off based on the desired reheat. This is a much simpler approach to controlling part load reheating. WAHX split loop systems are more cost effective as the number of individual heat pipes increase in number in a pipe to pipe WAHX system.
In review, all three systems provide unique approaches to part load reheat control of WAHX systems. Please contact us at ACT to review the best WAHX system for your air handler and start saving energy and dollars today. We are always available for discussions on payback analysis, but wanted to offer a basic understanding of how these systems can be an economic benefit rather than a regulatory burden!