T he United Arab Emirates (UAE) has one of the highest wet-
bulb design temperatures in the world (87°F [30.56°C]),
making it one of the most challenging places for controlling
indoor relative humidity. Around 20% of the total building
cooling load and annual energy consumption is used for the
treatment of the fresh air supply needed for ventilation. For
those reasons, we are always challenged to look for better
and more efficient ways to treat the fresh air supply.
By George J. Berbari, Member ASHRAE; Sleiman Shakkour, Member ASHRAE; & Fadi Hashem, Associate Member ASHRAE
George J. Berbari is CEO at DC PRO Engineering in the UAE. Sleiman I. Shakkour is district energy specialist at FVB Energy in Woodbridge, ON, Canada. Fadi Hashem is assistant mechanical engineering manager at DC PRO Engineering in the UAE.
In 1993 in a seminar held in the UAE, a cooling coil with runaround coil was presented as one possible solution. Since then the authors have helped introduce thousands of such systems into the local market and demonstrated in practice the effectiveness of this method of control-ling the indoor relative humidity.1
Later, we discovered the double heat
recovery unit, which after investigating thoroughly and having been convinced of its merits, introduced one of the first such system in the Middle East in 1999. Since then, we have promoted the benefits of using this method to treat the fresh air and hundreds of units have been installed and specified in our area. The use of this system has resulted not only in improved
overall efficiency but also proved effec-tive for controlling the indoor conditioned space relative humidity.
The increased supply of outdoor air needed for ventilation to achieve those acceptable indoor air quality levels rec-ommended in ANSI/ASHRAE Standard 62-1989, Ventilation for Acceptable In-door Air Quality, significantly increases the cooling and heating loads that the HVAC equipment needs to handle, result-ing in higher initial and operating system costs. To compensate, system designers look for innovative ways to reduce the energy consumption associated with the treatment of the supply of fresh air.
cycle for six different types of supply air-handling units was made to evaluate the available system design options to com-pare the impact of each method of treatment on capital costs and energy costs. This was used to establish the individual system merits for use as a guide when considering options for optimizing performance.
This article offers practical design guidelines based on our experience installing and maintaining fresh air makeup systems that include energy recovery.
Six Fresh Air-Handling UnitsThe six units use different air-to-air energy transfer technolo-
gies and are used to precondition outdoor air before it is mixed with return air from the conditioned space (Figure 1).
A. Conventional fresh air-handling unit with cooling coil and supply fan. The cooling coil dehumidifies the air to a constant 56°F (13.3°C) dew point, which is equivalent to the desired indoor condition of 76°F (24.4°C) dry bulb and 50% RH, without reheating it to a “neutral” dry-bulb temperature.
B. Double wheel energy recovery unit with a total recovery wheel and a sensible heat recovery wheel. This unit has two energy transfer stages between adjacent fresh air sup-ply duct and exhaust air duct, with air flowing at opposite directions, creating a counterflow heat exchange arrange-ment. In the first recovery phase, total energy exchange combining both latent and sensible energy transfer is achieved by virtue of a revolving enthalpy wheel (total effectiveness = 80%) having an aluminum backbone structure with a desiccant coating and large internal
surface area to transfer moisture and heat between the two airstreams having different temperatures, and vapor pressures as the driving force for energy transfer.
During the cooling season, this precools and dehu-midifies the fresh air prior to entering the main cooling coil, reducing the cooling load demand. The cooling coil dehumidifies the air to a constant 56°F (13.3°C) dew point. Leaving the cooling coil, this dehumidi-fied fresh air enters a second sensible-only wheel (sensible effectiveness = 70%), where it absorbs heat from the exhaust airstream and is reheated to a neutral air condition of 70°F (21.1°C) dry bulb. During this last stage, by releasing heat to the supply airstream, exhaust air is precooled prior to entering the first enthalpy wheel, which further enhances cooling the fresh airstream.
C. Fresh air-handling unit with total energy wheel and runaround coils. In this unit, a total energy wheel is placed in series with a runaround recovery coils (sensible effectiveness = 70%) placed between supply and exhaust air ducts. A pump circulates water that is used as a sensible energy transfer medium between the airstreams.
D. Fresh air-handling unit with total energy wheel and heat pipe coils. In this unit, heat pipe coils (sensible effective-ness = 63%) are placed between the supply and exhaust air ducts using refrigerant as the energy transfer medium, eliminating the need for a circulating pump.
E. Fresh air-handling unit with total energy wheel only. In this unit, an enthalpy wheel (total effectiveness = 80%)
36 ASHRAEJou rna l ash rae .o rg Janua ry 2007
Fresh Air
1 – Prefilter 2 – Bag Filter3 – Cooling Coil 4 – Supply Fan
12
34
Treated Fresh Air
Fresh Air-Handling Unit
Chilled Water45°F/55°F
Fresh Air
Bag Filter
Purge Air
Exhaust Air
Return Air
Supply Air
Purge Air
Total Energy Recovery
WheelOptional
Variable Speed Control
Cooling Coil Sensible Energy
Recovery Wheel
2 in.Filter
Fresh Air-Handling Unit With Double Recovery Wheels
Chilled Water45°F/55°F
2 in. Prefilter
Purge Air
Exhaust Air
Duct
FreshAir Duct
Total Energy Recovery
WheelOptional
Variable Speed Control
Cooling Coil
RunaroundCoil
2 in.Filter
Fresh Air-Handling Unit With Total Energy Recovery Wheel and Runaround Coil
Optional Three-Way
Valve
Exhaust Air
Fresh Air
C
B
D
A
Total Energy Recovery
WheelOptional
Variable Speed Control
Exhaust Air
Fresh Air
Purge Air
Optional Three-Way Valve
FreshAir Duct
Chilled Water45°F/55°F
Cooling Coil Heat Pipe 2 in.Filter
EChilled Water
45°F/55°F
Purge Air
FreshAir
Total Energy Recovery WheelOptional Variable
Speed Control
Exhaust Air
Duct
Supply Air
Return Air
Supply Air
Return Air
Purge Air
Exhaust Air
Fresh Air
2 in.Filter
Cooling Coil With Horseshoe Heat Pipe
F
Fresh Air-Handling Unit With Total Energy Recovery Wheel and Heat Pipe
Fresh Air-Handling Unit With Energy Wheel Only Fresh Air-Handling Unit With Total Energy Wheel And Horseshoe Heat Pipe
Chilled Water45°F/55°F
Exhaust Air
is added to precool and dehumidify the air entering the conventional fresh air-handling unit. Again, the cooling coil dehumidifies the air to a constant 56°F (13.3°C) dew point, without reheating it to a “neutral” dry-bulb temperature.
F. Fresh air-handling unit with total energy wheel and a
Figure 1: Different arrangements for fresh air-handling units.
2 in. Prefilter
Bag Filter
2 in. Prefilter
Bag Filter
2 in. Prefilter
Bag Filter
2 in. Prefilter
Bag Filter
Total Energy Recovery WheelOptional Variable
Speed Control
horseshoe heat pipe wrapped around the main cooling coil. In this arrangement, a precooling heat pipe coil (sensible effectiveness = 54%) is placed at the inlet to the main cooling coil to further precool the on-coil fresh air and reheat heat pipe coil is placed at the outlet to reheat fresh air to the desired condition.
2 in.Filter
38 ASHRAEJou rna l ash rae .o rg Janua ry 2007
Outdoor Conditions Total Energy Wheel Cooling Coil Sensible Wheel Supply Exhaust Supply Air Sensible Total Supply Exhaust Exhaust Air Outlet Air Inlet Outlet Capacity Capacity Air Outlet Air Inlet Air OutletTdb Twb BIN Tdb Twb Tdb Twb Tdb Twb Ton Ton Tdb Twb Tdb Twb Tdb Twb °F °F Hours °F °F °F °F °F °F °F °F °F °F °F °F
Table 1a: Double wheel heat recovery with total energy wheel and sensible energy wheel (10,000 cfm supply/exhaust model).
Table 1b: Total energy wheel with horseshoe arrangement (10,000 cfm supply/exhaust model).
Based on these selections, and by computing the air conditions at different sections of each unit, the annual cooling energy and electrical energy consumption were calculated using the Abu Dhabi bin hour data that were computed using weather data covering a span of 10 years, which were provided by the Abu Dhabi Ministry of Communications—Meteorological Depart-ment for the period 1985 – 1994.
Tables 1a and 1b show an example of a calculation for the cool-ing energy demand of the double-wheel energy recovery unit and
Cost and Energy Consumption Comparison To compare these units and decide which is the most ad-
vantageous to use, it is necessary to consider the cost of the equipment and its impact on the associated chiller plant cost, and the annual energy consumption cost. For this purpose, a 10,000 cfm (4720 L/s) model was chosen for comparison, and it was assumed that the fan brings this amount of outdoor air for 24 hours/day, every day of the year. Different selections and quotations were obtained from various major manufacturers.
Janua ry 2007 ASHRAEJou rna l 39
total energy wheel with horseshoe arrangement. Design condi-tions are shown in the first row according to ASHRAE climatic design conditions. Air conditions are obtained at each section of the unit, and the total annual cooling energy consumption was determined in that manner. It is assumed that the unit is cycled off when the ambient temperature drops below that required for indoor comfort conditions in accordance with ANSI/ASHRAE Standard 55-1992, Thermal Environmental Conditions for Hu-man Occupancy, at 76°F (24.4°C) and 50% RH.
Electrical energy was calculated taking into account the power consumption of the supply fan, the exhaust fan, the runaround pump, and the total energy and sensible wheel’s energy. Total energy was calculated based on the operating hours for each unit.
Table 2 shows a summary for the electrical energy consump-tion calculation. Two cases were considered for the dual energy recovery units with runaround coils and heat pipe options:
1. Face velocity through the coils 315 fpm (1.6 m/s); and2. Face velocity through the coils 510 fpm (2.6 m/s).Because of the large size of the energy wheel, enough cross-
sectional area is available for the coils, which helps reduce the face velocity and consequently the energy consumption of the fan, but for the cost of a bigger coil. Thus, these two options were considered to study the feasibility of increasing the coil’s size for reducing energy consumed.
Table 3 shows a summary of results for the cooling coil design load, annual cooling energy and electrical energy required for each unit.
Table 4 shows a comparison between the units considered. For comparison, the cost of the air-cooled chiller plant being considered was estimated at $1,200/ton ($341/kW), including civil, mechanical, electrical, and utility connection costs. The equipment costs were obtained from manufacturers’ quotations. Electrical consumption was assumed to be 1.7 kW/ton (6 kW/kW) for the total chilled water plant (including chillers, pumps and auxiliaries). The electric power cost was determined based on a flat rate of $0.0543/kWh. The total capital and operating costs were calculated, for a 20-year period. The net present value was determined using an 11.75% discount rate.
As shown on Table 4, the fresh air-handling unit resulted in the highest life-cycle cost. This value was reduced by 58% when the total energy wheel with the horseshoe heat pipe arrangement was used, which resulted in the lowest life-cycle cost. However, the drawback when using this unit is the difficulty of controlling the temperature and moisture content of the supply air, which will vary depending on the ambient conditions.
The life-cycle cost of the system with the dual energy re-covery unit, compared closely with that having the lowest life-cycle cost, but having the added advantage of supplying air at constant temperature and humidity levels year-round regardless of the outdoor conditions. This results in a better control of the indoor humidity levels.
The higher coil face velocity resulted in an increase of the life-cycle cost by about 8%. This lead into concluding that exceeding a coil face velocity of 400 fpm (2 m/s) is not rec-ommended.
Supply Supply Exhaust Exhaust Energy Sensible Runaround Total Equipment Fan Fan Fan Fan Wheel Wheel Coil Pump Power kW kWh kW kWh kWh kWh kWh kWh
Table 2: Electrical consumption for 10,000 cfm model.
40 ASHRAEJou rna l ash rae .o rg Janua ry 2007
We noticed that Unit A (conventional fresh air-handling unit) and Unit E (fresh air-handling unit with total-energy wheel only) both dehumidify the outdoor air to the same dew point as the other systems, but they deliver it cold—56.3°F (13.5°C) dry bulb—rather than reheating it to a neutral 70°F (21.1°C). In these two systems, this cold air is able to offset a portion of the space sensible cooling loads.
However, many designers don’t take into consideration the cooling effect for sizing the secondary (local) HVAC systems nor is it used to reduce the chilling plant size. In this case, it has no impact on the capital cost. In off peak hours, when the room temperature condition is satisfied and the secondary system is turned off, this cool, conditioned outdoor air may overcool the space. At such time, the dehumidified outdoor air should be reheated or the local HVAC equipment needs to add heat to avoid overcooling the space. For this analysis, we decided to list the cooling energy in the tables but to ignore its impact. The reader may choose otherwise.
RecommendationsBased on the results and assumptions of this specific analysis:• When no need exists for a constant supply temperature
and RH, the total energy wheel with a horseshoe heat pipe arrangement with the lowest energy consumption costs and
capital costs can be used (supply temperature was found to vary between 66°F and 70°F [18.9°C to 21.1°C] and RH between 59% and 67%). The recommended spacing between the heat pipe coils is 4.6 ft (1.4 m) for easier cooling coil’s maintenance, although this leads to having a slightly longer unit.
• Double energy recovery systems resulted in a better hu-midity control with a constant fresh air supply temperature and RH year-round regardless of fresh ambient conditions (70°F [21°C] and 61% RH). Although costs are higher, they are recommended for use when constant supply tem-perature and humidity are necessary.
Of course, the results of the analysis may differ with climate, operating hours, utility costs, and installed costs.
Design Guide for Fresh Air-Handling Unit The following are summarized design parameters recom-
mended based on the preceding analysis and our experience installing and maintaining these systems.
• Total energy or sensible wheel to have a maximum air face velocity of 600 fpm (3 m/s). This limits the pressure drop, blower power and cross leakage to modest levels.
• Heat pipe and runaround coil to have a maximum air face velocity of 400 fpm (2 m/s):
Design FAHU Coil Total Coil Additional Additional AHU Electric
Equipment Condition Cooling Capacity Cooling Energy Cooling Effect Cooling Effect Power & Exhaust
Fan Power db/wb, °F Tons Ton-Hours/Year Tons Ton-Hours/Year kWh/Year
Table 3: Comparison of different AHU types (10,000 cfm supply/exhaust model).
Janua ry 2007 ASHRAEJou rna l 41
As the area defined by the wheel allows larger rectan-gular coil area;
Heat pipe and runaround coils typically have lower ef-fectiveness than sensible wheel at same face velocity, for runaround coil or heat pipe (typically eight rows or less); and
Life-cycle analysis justifies use of lower face velocity.• Wheels and heat pipes should be tested and rated according
to the following:ANSI/ARI Standard 1060-2001, Performance Rating
of Air-to-Air Heat Exchangers for Energy Recovery Ventilation Equipment; and
ANSI/ASHRAE 84-1991, Method of Testing Air-to-Air Heat Exchangers.
• Recommended maximum air duct velocity to be 1,200 fpm (6 m/s) and total external air static pressure drop should not exceed 1.5 in. w.g. (380 Pa) for each of the fresh air and exhaust air ductwork. It is recommended to use the static regain method for duct sizing.
• Recommended maintenance spacing between wheel, coils, heat pipe and fans is 1.7 ft to 2 ft (500 to 600 mm).
• The energy wheel edges should be protected with an epoxy coating (or equivalent) to eliminate edge corrosion.
• Proper filtration should be provided at the fresh air intake
of the wheel as well as the exhaust air intake for proper wheel operation and for reducing the need for frequent cleaning and maintenance.
• The wheel purge system should be field adjusted to get the design purge air based on actual field differential air pressure between fresh air and exhaust airstream. The consultant or designer should specify air pressure taps extended to the unit’s outer panel to allow measurement of differential air pressure between upstream fresh air and upstream exhaust air of the wheel.
• An optional speed detector with alarm function and inter-face to the building management system is recommended to guard against motor or belt failure.
Controls for Fresh Air-Handling UnitNecessary Controls
• It is necessary to control the leaving air temperature from the cooling coil to a dry-bulb temperature of 56.3°F (13.5°C), which corresponds to the absolute humidity level of the comfort indoor condition of 76°F (24.4°C) and 50% RH.
• Fresh air fan motors start/stop, exhaust air fan start/stop , total energy wheel motor start/stop, sensible wheel motor start/stop or runaround pump start/stop with necessary
Table 4: Capital cost and life-cycle analysis (air-cooled chiller plant).
Equipment FAHU Chilled Outdoor Air Total Total Annual 20 Years Cooling Water Plant AHU & Exhaust Capital Consumption Life Cycle Capacity Capital Cost Fan Capital Cost Cost Cost Cost NPV Ton
electric protection and allowance for local as well as remote controls and communication.
• Ambient dew point (or grains transmitter) sensor to shut off the cooling coil motorized valve and enthalpy wheel motor when the ambient dew point is below 56°F (13.3°C).
Optional Control• Should the occasional need arise to control the final leaving
supply fresh air temperature (which, if the component is properly selected, should be achieved automatically), then the consultant or designer can opt to control the capacity via a variable speed drive of the sensible wheel motor, a solenoid valve for the heat pipe and a three-way bypass valve at the runaround coil.
• Variable speed drives for both fresh air and exhaust air can be adopted for variable occupancy applications such as office buildings, theaters, conference rooms, sport arenas, restaurants and others that are possible to control via CO2 sensors located in the exhaust air ducts.
Installation and Maintenance• Adjust the purge to actual site conditions.• Multipass labyrinth seals or adjustable brush seals are
important elements for effective and efficient wheel opera-
tion to minimize leakage rate to a range of 0.05% to 0.2%. Seals require field adjustment.
• Wheel media cleaning can be done annually using vacuum or pressurized air (hot water is accepted by some manufacturers). The wheel is designed for laminar flow and resists plugging and accumulation of dust particles because of the back-flushing done by the incoming and outgoing of airstreams, which help minimize the need for frequent wheel cleaning.
• Other wheel components requiring routine maintenance involve bearing lubrication, motor and gear reducer lubrication, verifying bearing bolt and sheave tight-ness, belt condition, rotor runout and flatness, media tightness, etc.
• Coils and heat pipe require minimum maintenance such as scheduled cleaning and runaround pump maintenance.
References1. Berbari, G.J. 1998. “Fresh air treatment in hot and humid cli-
mates.” ASHRAE Journal 40(10):64–70.
Bibliography2001 ASHRAE Handbook—Fundamentals.Selection and Pricing Software from SEMCO Inc. Heat Pipe and
