A correctional facility in western Massachusetts has eight (8) Roof Top Air Handling Units (RTUs), circled in red, that conditions the air in the facility. These eight RTUs, in addition to recirculating a significant amount of air, also bring in 4,000 cfm of fresh air and exhaust 4,000 cfm of return air to maintain favorable indoor air quality.
ACT was asked to review the RTU design and layout and propose a modification enhancement that would allow for recovering energy from the exhausted air stream and deliver that energy to the fresh air being drawn into the RTU. By doing so, the warm exhaust air in the winter can preheat the cold outside air and the cool exhaust air in the summer can pre-cool the warm outside air. Preheating and pre-cooling the outside air before it is conditioned by the RTU saves a tremendous amount of energy.
The Challenge
In this environment, there is limited access to the roof for even routine maintenance, so the facilities management wanted to use a passive heat exchanger that did not have any moving parts (belts, motors, bearings, etc). Furthermore, fixed plate type heat exchangers are very large and bulky, making it difficult and expensive to retrofit onto an existing unit without significant amounts of additional ductwork.
The Solution
Due to the relative ease of retrofitting, an ACT-AAHX (Air-to-Air Heat Pipe Heat Exchanger) was selected for this application over enthalpy wheels and plate-type heat exchangers. The thin, compact heat pipe heat exchanger was the best fit for the application. ACT- AAHXs were evaluated that utilized six (6) to ten (10) rows of heat pipes. AAHXs with six (6) rows of heat pipes are typically over 50% effective and have a relatively low-pressure drop (less than 0.5 IWG). Adding rows does increase the effectiveness; however, it is at the expense of an additional pressure drop. In this application, it was decided to select the six (6) row AAHX so that the existing fans/blowers did not have to be replaced with higher horsepower ones. The ACT-AAHX is designed to recover up to 47,500 BTU/hr in the summer and 162,500 BTU/hr in the winter.
An AAHX that is mounted perfectly level will work equally well in both directions (summer and winter mode). However in this case, because the AAHXs were fairly large and long (7.5 ft long), a tilt mechanism was provided for each AAHX. In summer, the AAHX tilts such that the warm outside air passes through the heat pipe heat exchanger’s lowest side. In this orientation, the warm outside air causes the heat pipe working fluid, R-134a, to vaporize and travel to the higher end of the AAHX. Here the cool exhaust air causes the vapor to condense, giving up its latent heat of vaporization. The condensed liquid R-134a then returns to the lower side by gravity flow, where it is available for vaporization again. In the winter, the AAHX is tilted the opposite direction. The amount of tilt is approximately one inch.
The Results
ACT’s tilting AAHX can be installed in a similar manner to a standard AAHX because the tilting AAHX has a stationary outer casing that can be securely attached to duct-work. Only the internal energy recovery casing moves. The tilt mechanism can also be used to prevent unwanted preheating during shorter days in the spring and fall seasons when the RTU is in the cooling mode but the outside air temperature temporarily drops below the exhaust air temperature (typically overnight).
Furthermore, the tilt mechanism can be used to address frosting issues. Under some conditions, extremely cold outside air and high humidity exhaust air, the exhaust air stream will start to condense water. This water can freeze and block the flow of air through the AAHX preventing it from recovering energy as designed. In this case, the tilt mechanism can be used to position the AAHX in a non-functioning orientation. The warm exhaust air, that is no longer being cooled, will defrost the exhaust side of the coil. A simple algorithm built into the building management system can be used to manage the shorter day cycle and defrost cycle automatically.
Payback on dual-season Air-to-Air Heat Pipe Heat Exchanger Systems with relatively long cold winter seasons, such as the New England area, will typically payback in under 1.5 years.
