After all these years... It's Still Efficient
Approximately five years ago, the Fox Chase Cancer Center finished the construction of a $30 million, four-story, 112,000-sq.-ft. research facility known as the Cancer Prevention Pavilion. Located in northeast Philadelphia, the Fox Chase Cancer Center's mission is to "reduce the burden of human cancer through the highest-quality programs in research and patient care, including cancer prevention, treatment, early detection and education."
The Pavilion was designed with an innovative air-handling unit (AHU) and heat pipe energy recovery system. Laboratories on the second and third floors utilize a once-through, 100% outside air system. The supply and exhaust airstreams for these spaces run side by side through a heat pipe energy recovery device located in the fourth-floor mechanical penthouse.
Installed within the two laboratory central AHUs, the heat pipe recovery unit consists of a bank of multiple independent refrigerant-filled tubes that transfer thermal energy between two independent airstreams. Advantages that the unit has over other types of recovery devices include a minimum of moving parts and a bypass section to control recovery capacity. It also offers built-in redundancy, as the failure of one of the 120 tubes would only marginally affect performance.
The AHUs housing the heat pipe consist of a supply unit with pre-filters, heat pipe, preheat coil, cooling coil, humidifier, fan section, sound attenuators and final filters. The exhaust units contain filters, heat pipe and fan section. Whereas this configuration requires the use of a custom AHU for both supply and exhaust airstreams, other forms of recovery devices, such as plate-type exchangers or run-around-type coils, would have imposed high sheet metal costs, space constraints and added pumping and piping costs. Wheel-type heat recovery was rejected in this application due to the concern of cross-contamination between the airstreams. This is prevented in the heat pipe system by separating airstreams with a sealed partition.
The primary benefit of the heat pipe system is a high thermal heat transfer effectiveness of approximately 65% at the coil velocities. The units are also virtually maintenance-free, since there are no mechanical or electrical inputs required, other than a simple bypass damper system. The pipes are filled with R-22 or R-134a refrigerant that performs a "siphoning" effect between the two airstreams, thereby transferring thermal energy from one to the other. The refrigerant operates continuously in a vacuum within each coated aluminum tube that unless punctured, will outlast the AHU in which it's installed. This thermal energy transfer is continuous without the consumption of any outside energy.
Fox Chase is satisfied with all aspects of the heat pipe and is particularly enthusiastic about the indirect evaporative cooling part of the system. One of the most powerful applications of the heat pipe is water-spray evaporative cooling, also known as indirect evaporative cooling (IDEC). IDEC is a way of capturing most of the cooling energy normally lost when conditioned air is exhausted from a building. It is also a way of cooling building make-up air without adding humidity.
The 100% outside air-handling units cool the incoming air by using the psychometric potential of air exhausted from the building. Water is sprayed on the exhaust air side of the heat pipe, lowering the airstream temperature toward its wet-bulb temperature. With the lab at 75°F and 50% relative humidity, we expect the airstream at the heat pipe coil to be 78°F dry bulb and 65°F wet bulb. The heat pipe then transfers this additional thermal energy to the supply air stream without adding humidity. This transfer takes effect via the evaporative cooling process that depresses the 78°F dry-bulb exhaust temperature much closer to the 65°F wet-bulb temperature.
The result is a significant drop in mechanical air-conditioning requirements and a significant reduction in electric power consumption. It is this power consumption reduction that is magnified in higher outside air temperatures, thereby reducing the peak demand of the central chilled water plant and the utility peak demand or charge by more than 100 tons on a design (93°F) day. (100 tons of cooling at about 0.7 kW/ton equals approximately 70 kW of reduction.)
In August 2001, data taken from the direct digital control system at Fox Chase indicated that with a peak outside air temperature between 80°F and 90°F, the air that entered the cooling coils as a result of the heat pipe was between 12°F to 20°F less!
With results such as these, the client has been quite happy with the system and has enjoyed the benefits of chilled-water savings and the associated electrical demand savings from its chilled water plant. The system has required minimal maintenance to date. No tubes are known to have failed and the only maintenance required is an occasional coil cleaning as a result of using the water spray/IDEC system. That being said, I've learned that there have been some problems with the pumps, and efforts are underway to resolve the issue.
A boiler system that requires softened water is located within the same penthouse as the heat pipe system. Since boiler make-up for the humidifiers was minimal in the summer, we utilized this soft water for the indirect evaporative cooling system designed for the heat pipes. One difficulty on start-up of the heat pipe was setting up the nozzles that spray this softened water onto the heat pipe for the IDEC effect.
Occasionally, the nozzles clog from a salt sediment buildup; a discharge solenoid valve even became clogged once. The coil also required a cleaning about every two years from a slight salt sediment buildup. Fox Chase is currently considering using an evaporative pad, in lieu of the spray nozzles, that would be placed in front of the heat pipes and which could receive non-softened water. This pad would not require any nozzles; water would flow down from the top and cleanup of the coils would be minimized since no water would actually hit the heat pipe coils. End-of-season cleanup would entail the replacement of this pad. Salt sediment is more easily cleaned from the coils but water hardness/calcium buildup requires much more strenuous cleaning. Non-softened water could be used since no water is actually hitting the coils.
In future heat pipe installations, the use of an evaporative pad would be beneficial as it would eliminate nozzle problems and not require softened water.
Heat pipe evolution
When the heat pipe system was designed, Fox Chase required that the supply and exhaust airstreams be laid out side by side. A major heat pipe manufacturer, Heat Pipe Technology, Gainesville, Fla., has since designed, installed and operated, for at least three years, a "split" or "run-around" heat pipe.
The heat pipe has the benefit of greater heat transfer and associated energy savings over the normal run-around glycol coil systems. The big advantage of a glycol run-around system is that it does not require the airstreams to be laid out in a side-by-side configuration. With the new split heat pipe system a designer can get the best of both worlds.
The split heat pipe is a refrigerant-based run-around coil system. There are some limitations in distance between the two heat pipe coils, but the advantages of this system are tremendous. Retrofit applications with higher energy efficiencies are now within grasp. The system utilizes fractional horsepower pumps to move lightweight refrigerant instead of large multi-horsepower pumps moving a heavy glycol mixture. So with higher heat transfer rates and the potential for additional savings from the indirect evaporative effect, there are, and will continue to be, opportunities for exciting new applications.