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 a 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 precool the warm outside air. Preheating and precooling the outside air before it is conditioned by the RTU saves a tremendous amount of energy.
The Challenge: 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. 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 does 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 duct work. 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 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 shoulder days in the spring and fall when the RTU is in 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 shoulder day cycle and defrost cycle automatically.
Payback on a dual-season AAHX systems with relatively long cold winter seasons, like in New England, will typically payback in under 1.5 years.
Wrap-Around Heat Pipe for Enhanced Dehumidification and Energy Savings in a Multi-Story Condominium – Retrofit
Project Overview: Three, twenty story, luxury condominiums on the southeast coast of Florida. Each tower with its own rooftop 100% Dedicated Outside Air System (DOAS) Make-up Air Units (MUA). These units provide cool fresh air to the hallways and corridors of the buildings. The MUAs bring in 100% outside air, at approximately 17,000cfm. The air is cooled down to 55°F to remove the moisture from the air. Since the air conditioning load for the hallways is low (residents only pass through and the solar load is kept low through good architectural design), the air is then reheated to approximately 65°F to prevent the hallways from becoming too cool. This exiting systems reheat is provided by the direct expansion (DX) vapor compression system through a hot gas reheat coil. The roof top MUA units are AAON RN series units.
The Challenge: ACT was brought in to conduct an analysis on the MUAs to determine if ACT’s wrap-around heat pipe heat exchangers (WAHXs) could save operating costs by reducing the cooling load (tons of cooling savings) on the active cooling coil and supplying some of the reheat.
Local historical weather data was gathered and a BIN analysis was performed. The weather data was divided into dry-bulb temperature groups in 5°F intervals. ACT’s wrap- around heat pipe selection tool http://www.1-ACT.com/HVAC/WAHX was run at each of these temperature points to determine the tons of cooling savings and the reheat reduction. The wrap-around units utilized coils that were 3 rows deep at 12 fins per inch. The results were totaled for the year (sum of savings/hour multiplied by the number of hours per year at that condition). Those results, along with the local electricity rate and the conservatively estimated coefficient of performance (COP) of the AAON units, were used to determine the annual savings of approximately $7,000 per year. No savings credit was taken for the reheat because the units were utilizing compressor hot gas reheat. At this level of savings per year, the payback period for retrofitting ACT –WAHXs was on the order of three (3) years.
Project Details: The retrofitting process began by moving the DX evaporation coil inlet and outlet tubes (supply manifolds) and the hot gas reheat coil to make room for the additional WAHX coils. This work was done by a local commercial HVAC contractor hired by and paid by ACT as part of this job (it was a planned cost, not an additional unforeseen cost). ACT then installed the pre-cool and reheat coils into the units and cut openings in the wall of the AAON units for the WAHX connecting tubes to pass through. The connecting tubes were brazed and the heat pipes were evacuated and charged on site. A drain pan was added under the precool coil of the WAHX to collect any condensation that should occur on that coil during particularly humid times of the year. A new filter rack was also fabricated and installed in front of the pre-cool coil (another planned cost).
The entire job was managed by ACT with little disruption to the daily operation of the condominium. All three units were retrofit with WAHXs by ACT technicians. The job took approximately two weeks from start to finish with minimum downtime on the units.
The Condo Management Group also requested a means to verify the performance and savings. ACT provided an Energy Monitoring System (ACT-EMS) that calculates and records the daily, monthly and annual energy savings based on actual MUA operating conditions. This information can be viewed at the air handler or remotely via the web. The WAHX performance has met the original design goals and on warm, 90°F summer days, 10 degrees of pre-cooling is achieved by the ACT WAHX units.
Tucked away in the West Virginia Mountains next to the slow flowing Ohio River is the CertainTeed drywall facility.
The Moundsville, WV plant is designed to flow drywall sheet materials up to 500 feet per minute. The process pours, shapes, flattens, and dries the product to its final form. Any part of the production process that produces waste is recycled. Even the materials that make up the product are derived from local waste and recycled material streams.
A major element in the manufacturing process is water, which must be controlled within a tight temperature range. Process water that is too hot or too cold results in product inconsistencies and poor quality.
The water driven off during the drying process is condensed and the temperature is adjusted by high capacity cooling towers. Unfortunately, during the hot summer, the temperature of the reclaimed water exceeds the maximum allowable range, resulting in unscheduled and unwanted plant shutdowns. To prevent this, supplemental cooling capacity was required to lower the process water temperature.
Application: ACT was contacted to evaluate and size a large, direct drive, fan cooled radiator system that could be placed in series with the building products process water loop. ACT provided performance predictions as a function of flow rate, incoming process water temperature, outdoor temperature, and fan speed. The water is continuously pumped through the radiator at a fixed flow rate between 17 and 21 gpm. The temperature of the water leaving the fan cooled radiator is controlled by varying the speed of the fan. A variable frequency drive was used to modulate the fan motor speed. The process control parameters are monitored and controlled by local PLCs and coordinated through the plant SCADA system. The fan cooled radiator system was required to cool the process water to approximately 60°F, which translates into a cooling capacity of over 600,000 BTU/hr. of cooling.
Design Scope: The fan controlled radiator was engineered to address the process parameters. All metal surfaces were formed from 304 stainless steel. The radiator coils were E-coated to prevent corrosion. A direct drive 54 inch diameter, 10.9° pitch, high speed fan was driven by a ten horsepower AC motor. An Ethernet based temperature sensor is used to provide input to a PLC. The PLC communicates via Allen Bradley Device Net to vary the frequency of the 460VAC motor drive coupled to the fan cooled radiator’s 10 HP motor.
Conception to Installation and Start-Up: ACT provided support throughout the entire project ranging from the initial design to predictive performance modeling to field installation and startup support. ACT also designed and built the custom three foot high, galvanized steel mounting base and the AC drive motor control cabinet. The final deliverable included full system documentation including startup and maintenance recommendations.
Project Results: The plant engineering and facilities teams are pleased with the initial performance of the radiator system. Early in the conceptual stage, a $400,000 chiller system was proposed. The chiller system was expensive from both initial cost and operating cost points of view. The fan cooled radiator solution from ACT was chosen over the chiller system as the solution with the highest value, value being defined as function / cost. A cost avoidance of nearly $400K was realized. Thomas Oei, Sr. Project Manager for CertainTeed commented on the project:
An effective, innovative and complete solution for our cooling need was provided by ACT.
Their continual support from project conception to commissioning resulted in exceeding our expectations as well as the original equipment specifications.”
The project was completed one week ahead of schedule and 15% under budget. It was in full compliance with CertainTeed’s World Class Manufacturing (WCM) practices, in large part because of the complete documentation and vendor information provided. CertainTeed also achieved “vertical startup”, which is one of the aims of WCM. Vertical startup means the switch is turned on and it works, and continues to work. Thomas Oei continued “In the opinion of CertainTeed, ACT was able to work well with our local contractors and plant staff to ensure the successful equipment integration into the plant automation systems.”
The additional process control achieved through the implementation of the supplemental fan cooled radiator will result in less downtime, improved quality, and will allow CertainTeed to do innovative new product development that may require cooler process water temperatures.
A well-established Asian air handling company wanted to provide cost saving energy recovery at a large KAL resort hotel. An air to air heat pipe heat exchanger (AAHX) was designed and built to pre-cool the incoming ventilation air with conditioned exhaust air. Up to 10,000 cfm of incoming outside air at 95°F was pre-cooled to 67.6⁰F prior to being processed by the active air conditioning system. The targeted temperature effectiveness was 80%. In other words, the AAHX was designed to transfer 80% of the temperature difference between the incoming air and the exhaust air streams.The overall size of the AAHX was approximately 6ft. wide x 8.5ft tall. ACT’s AAHX is installed vertically with the warm incoming airstream ducted through the lower section and the cooler exhaust airstream ducted through the upper section. The two airstreams are completely isolated, by a sealed partition, with no potential for cross contamination.
Energy Recovery: Based on the 80% sensible effectiveness, the incoming warm airstream can be pre-cooled from 95°F to 67.6°F. This reduction in entering air temperature generates a saving of over 295KBtu/hr at the system design point of 95° F. See the specific calculations below:
Effectiveness = 0.80 = 10,000 cfm * (95°F – X°F)/10,000 cfm * (95°F-60.8°F) X = 27.4° F
(warm stream ∆T) 95° – 27.4° = 67.6 °F
Sensible Air Cooling (Btu/hr) = 1.08*Flowrate (cfm)*∆T(°F) 1.08*10,000 cfm*27.4°F = 295,920 Btu/hr
Let’s convert the 295,920 Btu/hr energy savings to dollars. The hotel is located in a major metropolitan area, with an estimated cost of electricity at $0.15/kWh. At the design point, energy recovery savings are approximately $104.40/day. A 24 hour/day system operation daily savings calculation is shown below.
295,920 Btu/hr = Energy Savings 295,920 Btu/hr *.2931 W/Btu/hr / 3 (COP of active cooling system) ≈ 29 kW x 24hours/day *$0.15/kWh = $104.40/day
The estimated payback period for the AAHX heat pipe heat exchanger, depending on probable hours of operation, is under one (1) year.
Meeting the Design Specification:
ACT performed a prototype study to meet the design requirement of 80% temperature effectiveness. The physical dimensions of the heat exchanger, airflow rates, entering and exiting temperatures, and the maximum allowable pressure drop were provided by the customer. Parametric studies were run to determine the effectiveness as a function of number of rows, fins per inch, and fin type. Based on the analysis, a final design was selected that best met the customer requirements.
Additionally, a full scale demonstration heat pipe was built and tested to determine the optimum fluid charge for the heat pipe heat exchanger. The power transfer capacity and the associated heat pipe ∆T are a strong function of the fluid charge in long, vertically oriented heat pipes. The test pipe was charged with various amounts of R-134a refrigerant and the optimum charge was determined to achieve the highest level of energy transfer at the lowest ΔT.
The final design was an AAHX that contained 540 individual heat pipes, ten (10) rows deep, with aluminum corrugated fins spaced at 14 fins per inch. The pressure drop of the air through the heat pipe heat exchanger was approximately 1.0 inch of water. The ACT AAHX was delivered in August 2013.