Airflow Analysis of a Custom “Green” Air Handling Unit Without a Pre-heater Performed by M/E Engineering The CAES Group 22 June 2010
Background • Two years ago, a pharmaceutical client finished construction on a new LEED certified building in New England. • The facility’s air handlers were custom-fabricated and employed economizers without pre-heaters upstream of the water-based coils. • Without pre-heaters, the air handlers were much more energy efficient. • The air handler manufacturer believed that their design was capable of adequately mixing warm RETURN air with 30% cold OUTSIDE air effectively enough to prevent coil freezing. • During the first year of operation, freezestat alarms shut most of the units down during several cold winter days. • The client asked that we model their air handlers to: – Understand the issues with the current design, and – Provide them with a redesign that would be immune to cold climatic conditions.
Case 1: Parameters • Star CCM+ software was used to model an Economizer/Air Handling Unit installed on a LEED silver certified building in the pharmaceutical industry. • Given Values: – Air Flow CFM = 51,100 CFM – 30% Outside Air and Temperature = 0ºF – Supply Air Temperature Target = 55ºF • Assumptions: – Return Air Temperature = 70ºF
Case 1 Custom Air Handler without Pre-Heating Geometric Elevation View (Visualization Only) Actual CFD Model Supply Fan Return Fan Return Air Enters Humidifier Spray Nozzles Blenders Cooling Coils Outside Air Enters Pre-Filters
Case 1 Custom Air Handler without Pre-Heating Geometric Plan View Outside Air Enters Supply Fan Motor Supply Fan Blenders Humidifier Spray Nozzles Pre-Filters Cooling Coils
Case 1 Custom Air Handler without Pre-Heating Geometric Elevation View Outside Air Enters Return Air Enters Supply Fan Blenders Humidifier Spray Nozzles Pre-Filters Cooling Coils
Case 1 Custom Air Handler without Pre-Heating Geometric Isometric View Outside Air Enters Return Air Enters Supply Fan Humidifier Spray Nozzles Blenders Pre-Filters Cooling Coils
Case 1 Custom Air Handler without Pre-Heating Pathlines Isometric View
Case 1 Custom Air Handler without Pre-Heating Pathlines Plan View Blenders Pre-Filters Cooling Coils Humidifier Spray Nozzles
Case 1 Custom Air Handler without Pre-Heating Pathlines Elevation View Blenders Pre-Filters Cooling Coils Humidifier Spray Nozzles
Case 1 Custom Air Handler without Pre-Heating Temperature Contour – Isometric View Cut Plane approx. thru center of AHU
Case 1 Custom Air Handler without Pre-Heating Temperature Contour – Side Elevation View Cooling Coils
Case 1 Custom Air Handler without Pre-Heating Relative Pressure Contour – Side Elevation View ∆ P =0.23 in WG ∆ P =0.48 in WG P ≈ 0 in WG
Case 1 Custom Air Handler without Pre-Heating Velocity Vectors – Side Elevation View Blenders Pre-Filters Cooling Coils Humidifier Spray Nozzles
Case 1 Custom Air Handler without Pre-Heating Velocity Vectors – Close up Side Elevation View Blenders Pre-Filters Cooling Coils Humidifier Spray Nozzles
Case 1 Custom Air Handler without Pre-Heating Speed Contour & Velocity Vectors – Side Elevation View Blenders Pre-Filters Cooling Coils Humidifier Spray Nozzles
Case 1 Custom Air Handler without Pre-Heating Cooling Coil Face Temperature – Isometric View Blenders Pre-Filters Cooling Coils Humidifier Spray Nozzles
Case 1 Custom Air Handler without Pre-Heating Cooling Coil Face Temperature – Front Elevation View Cooling Coils (min & max values)
Case 1 Custom Air Handler without Pre-Heating Cooling Coil Face Temperature – Front Elevation View Cooling Coils (cold spots below 35°F)
Case 1 Custom Air Handler without Pre-Heating Cooling Coil Face Temperatures at Various Heights Bottom Coil Middle Coil Top Coil
Case 1 Custom Air Handler without Pre-Heating Pre-Filter Face Air Speed – Isometric View Blenders Pre-Filters Cooling Coils Humidifier Spray Nozzles
Case 1 Custom Air Handler without Pre-Heating Pre-Filter Face Air Speed – Front Elevation View Pre-Filters
Case 1 Estimated Supply Temperature with perfect mixing 70% Return & 30% Outside Air Estimated supply discharge temperature
Case 1 Conclusions • The model shows that there’s still a probability of the top cooling coil to freeze compared to a design without blenders. – The temperatures in the supply chamber range from 65ºF down to 26ºF, with an average discharge temperature of 44ºF. – A simple mixing model calculates the discharge temperature at 49.2°F. (assuming perfect mixing)
Case 2: Parameters • Star CCM+ software was used to model the Economizer. • Given Values: – Air Flow CFM = 51,100 CFM – 30% Outside Air and Temperature = 0ºF – Supply Air Temperature Target = 55ºF • Assumptions: – Return Air Temperature = 70ºF • Modified Blender location and interior wall.
Case 2 Custom Air Handler without Pre-Heating Geometric Elevation View Outside Air Enters Modified interior wall Return Air Enters Supply Fan New Location for Blenders Humidifier Spray Nozzles Pre-Filters Cooling Coils
Case 2 Custom Air Handler without Pre-Heating Geometric Isometric View Outside Air Enters Modified interior wall Return Air Enters Supply Fan Humidifier Spray Nozzles New Location for Blenders Pre-Filters Cooling Coils
Case 2 Custom Air Handler without Pre-Heating Temperature Contour – Side Elevation View Cooling Coils
Case 2 Custom Air Handler without Pre-Heating Cooling Coil Face Temperature – Front Elevation View Cooling Coils (min & max values)
Case 2 Custom Air Handler without Pre-Heating Cooling Coil Face Temperature – Front Elevation View Cooling Coils (cold spots below 35°F)
Case 2 Custom Air Handler without Pre-Heating Cooling Coil Face Temperatures at Various Heights 35° Bottom Coil Middle Coil Top Coil
Case 2 Estimated Supply Temperature with perfect mixing 70% Return & 30% Outside Air Estimated supply discharge temperature
Case 2 Conclusions – Case 2 • This Economizer model shows that there’s still a probability of the cooling coils to freeze. – The temperatures in the supply chamber range from 63ºF down to 26ºF, with an average discharge temperature of 43ºF. – Temperature distribution at coil faces was shifted to slightly higher temperatures.
Case 3: Parameters • Star CCM+ software was used to model the Economizer. • Given Values: – Air Flow CFM = 51,100 CFM – 30% Outside Air and Temperature = 0ºF – Supply Air Temperature Target = 55ºF • Assumptions: – Return Air Temperature = 70ºF • Moved the Blender down 5” from the floor.
Case 3 Custom Air Handler without Pre-Heating Geometric Elevation View Outside Air Enters Return Air Enters Supply Fan New Location for Blenders Humidifier Spray Nozzles (5” from floor) Pre-Filters Cooling Coils
Case 3 Custom Air Handler without Pre-Heating Geometric Isometric View Outside Air Enters Return Air Enters Supply Fan Humidifier Spray Nozzles New Location for Blenders (5” from floor) Pre-Filters Cooling Coils
Case 3 Custom Air Handler without Pre-Heating Temperature Contour – Side Elevation View Cooling Coils
Case 3 Custom Air Handler without Pre-Heating Cooling Coil Face Temperature – Front Elevation View Cooling Coils (min & max values)
Case 3 Custom Air Handler without Pre-Heating Cooling Coil Face Temperature – Front Elevation View Cooling Coils (cold spots below 35°F)
Case 3 Custom Air Handler without Pre-Heating Cooling Coil Face Temperatures at Various Heights 35° Bottom Coil Middle Coil Top Coil
Case 3 Custom Air Handler without Pre-Heating Cooling Coil Face Temperature – Front Elevation View Comparison of Surface Areas below 35°F Case 3 Case 1 Case 2
Case 3 Estimated Supply Temperature with perfect mixing 70% Return & 30% Outside Air Estimated supply discharge temperature
Case 3 Conclusions • This Economizer model shows that there’s still a probability of the cooling coils to freeze. – The temperatures in the supply chamber range from 58ºF down to 28ºF, with an average discharge temperature of 43ºF. • SUGGESTED DESIGN CHANGE – Place vanes beneath the outside air damper to skew the cold air flow rightward.
Case 4: Parameters • Star CCM+ software was used to model the Economizer. • Given Values: – Air Flow CFM = 51,100 CFM – 30% Outside Air and Temperature = 0ºF – Supply Air Temperature Target = 55ºF • Assumptions: – Return Air Temperature = 70ºF
Case 4: Parameters • Kept the Blender 5” from the floor • Added 9” deep vanes below the outside air damper. (6” gap between damper and top of vanes, half of the vanes are at 45º to the outside side walls with the middle vane placed vertically. All vanes are evenly spaced 1ft apart under the Outside Damper. • Added a perturbation airfoil that starts 6” above the opening of the blenders, protrudes 10 inches upstream and angles back at a 45º to the wall. This perturbation airfoil extends laterally wall to wall above the openings of the blenders.
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