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Airflow Analysis of a Custom “Green” Air Handling Unit Without a Pre-heater Performed by M/E Engineering The CAES Group 22 June 2010
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
Custom Air Handler without Pre-Heating Geometric Elevation View
(Visualization Only)
Return Fan Supply Fan
Actual CFD Model
Humidifier Spray Nozzles
Pre-Filters Cooling Coils Blenders
Return Air Enters
Outside Air Enters
Case 1
Custom Air Handler without Pre-Heating Geometric Plan View
Supply Fan
Humidifier Spray Nozzles Pre-Filters Cooling Coils
Blenders
Outside Air Enters Supply Fan Motor
Case 1
Custom Air Handler without Pre-Heating Geometric Elevation View
Supply Fan
Humidifier Spray Nozzles
Pre-Filters Cooling Coils
Blenders
Outside Air Enters
Return Air Enters
Case 1
Custom Air Handler without Pre-Heating Geometric Isometric View
Supply Fan
Humidifier Spray Nozzles
Pre-Filters Cooling Coils Blenders
Outside Air Enters
Return Air Enters
Case 1
Custom Air Handler without Pre-Heating Pathlines Isometric View
Case 1
Custom Air Handler without Pre-Heating Pathlines Plan View
Pre-Filters Cooling Coils Blenders Humidifier Spray Nozzles
Case 1
Custom Air Handler without Pre-Heating Pathlines Elevation View
Pre-Filters Cooling Coils Blenders 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
Pre-Filters Cooling Coils Blenders Humidifier Spray Nozzles
Case 1
Custom Air Handler without Pre-Heating Velocity Vectors – Close up Side Elevation View
Pre-Filters Cooling Coils Blenders Humidifier Spray Nozzles
Case 1
Custom Air Handler without Pre-Heating Speed Contour & Velocity Vectors –
Side Elevation View Pre-Filters Cooling Coils Blenders Humidifier Spray Nozzles
Case 1
Custom Air Handler without Pre-Heating Cooling Coil Face Temperature – Isometric View
Pre-Filters Cooling Coils Blenders 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
Cooling Coils (cold spots below 35°F)
Custom Air Handler without Pre-Heating Cooling Coil Face Temperature – Front Elevation View
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
Pre-Filters Cooling Coils Blenders Humidifier Spray Nozzles
Case 1
Pre-Filters
Custom Air Handler without Pre-Heating Pre-Filter Face Air Speed – Front Elevation View
Case 1
Estimated Supply Temperature with perfect mixing
70% Return & 30% Outside Air
Case 1
Estimated supply discharge temperature
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 1
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.
Custom Air Handler without Pre-Heating Geometric Elevation View
Supply Fan
Humidifier Spray Nozzles
Pre-Filters Cooling Coils
New Location for Blenders
Outside Air Enters
Return Air Enters
Modified interior wall
Case 2
Custom Air Handler without Pre-Heating Geometric Isometric View
Supply Fan
Humidifier Spray Nozzles
Pre-Filters Cooling Coils
Outside Air Enters
Return Air Enters
New Location for Blenders
Modified interior wall
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
Cooling Coils (cold spots below 35°F)
Custom Air Handler without Pre-Heating Cooling Coil Face Temperature – Front Elevation View
Case 2
Custom Air Handler without Pre-Heating Cooling Coil Face Temperatures at Various Heights
Bottom Coil Middle Coil Top Coil
35°
Case 2
Estimated Supply Temperature with perfect mixing
Case 2
70% Return & 30% Outside Air
Estimated supply discharge temperature
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 2
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.
Custom Air Handler without Pre-Heating Geometric Elevation View
Supply Fan
Humidifier Spray Nozzles
Pre-Filters Cooling Coils
New Location for Blenders (5” from floor)
Outside Air Enters
Return Air Enters
Case 3
Custom Air Handler without Pre-Heating Geometric Isometric View
Supply Fan
Humidifier Spray Nozzles
Pre-Filters Cooling Coils
Outside Air Enters
Return Air Enters
New Location for Blenders (5” from floor)
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
Cooling Coils (cold spots below 35°F)
Custom Air Handler without Pre-Heating Cooling Coil Face Temperature – Front Elevation View
Case 3
Custom Air Handler without Pre-Heating Cooling Coil Face Temperatures at Various Heights
Bottom Coil Middle Coil Top Coil
35°
Case 3
Case 3
Case 2
Case 1
Custom Air Handler without Pre-Heating Cooling Coil Face Temperature – Front Elevation View
Comparison of Surface Areas below 35°F
Case 3
Estimated Supply Temperature with perfect mixing
Case 3
70% Return & 30% Outside Air
Estimated supply discharge temperature
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 3
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
• 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.
Case 4: Parameters
Custom Air Handler without Pre-Heating Geometric Side Elevation View
Pre-Filters
Cooling Coils Outside Air Enters
Return Air Enters
Case 4
Blenders
Added Vanes
Perturbation Airfoil
Custom Air Handler without Pre-Heating Geometric Front Elevation View
Outside Air Damper
Return Air Damper
Case 4
Added 9” Vanes @ 45 º
Custom Air Handler without Pre-Heating Geometric Isometric View
Supply Fan
Humidifier Spray Nozzles
Blenders
Cooling Coils
Outside Air Enters
Return Air Enters
Case 4
Added Vanes
Pre-Filters
Perturbation Airfoil
Custom Air Handler without Pre-Heating Temperature Contour – Side Elevation View
Cooling Coils
Case 4
Custom Air Handler without Pre-Heating Cooling Coil Face Temperature – Front Elevation View
Cooling Coils (min & max values)
Case 4
Custom Air Handler without Pre-Heating Cooling Coil Face Temperatures at Various Heights
Bottom Coil Middle Coil Top Coil
35°
Case 4
Custom Air Handler without Pre-Heating Relative Pressure Contour – Side Elevation View
∆P =0.26 in WG ∆P =0.48 in WG
Case 4
∆P =0.33 in WG
P ≈ 0 in WG
Estimated Supply Temperature with perfect mixing
Case 4
70% Return & 30% Outside Air
Estimated supply discharge temperature
Conclusions • With the added vanes before the Blenders increases the cooling coil
face temperature up to 35ºF. • Still a small possibility that the cooling coils might freeze on the lower
right cooling coil.
• Recommend mirroring the blade rotation of the right blender to create a more even temperature distribution on the cooling coils.
Case 4
Case 5: 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 5: Parameters • Kept the Blender 5” from the floor • Kept the vanes below the outside air damper. • Kept the perturbation airfoil. • Mirrored the right blender (when looking downstream towards the
supply fan in the air handling unit.)
Custom Air Handler without Pre-Heating Geometric Side Elevation View
Pre-Filters
Cooling Coils Outside Air Enters
Return Air Enters
Case 5
Blenders
Vanes
Perturbation Airfoil
Custom Air Handler without Pre-Heating Geometric Isometric View
Supply Fan
Humidifier Spray Nozzles
Blenders Cooling Coils
Outside Air Enters
Return Air Enters
Case 5
Vanes
Pre-Filters Mirrored Blender
Perturbation Airfoil
Custom Air Handler without Pre-Heating Geometric Isometric View
Blenders
Outside Air Enters
Return Air Enters
Case 5
Vanes
Pre-Filters
Mirrored Blender
Perturbation Airfoil
Custom Air Handler without Pre-Heating Temperature Contour – Side Elevation View
Cooling Coils
Case 5
Custom Air Handler without Pre-Heating Cooling Coil Face Temperature – Front Elevation View
Cooling Coils (35º - max value)
Case 5
Bottom Coil Middle Coil Top Coil
Case 5
Custom Air Handler without Pre-Heating Cooling Coil Face Temperatures at Various Heights
Custom Air Handler without Pre-Heating Relative Pressure Contour – Side Elevation View
∆P =0.22 in WG
∆P =0.41 in WG
Case 5
∆P =0.40 in WG
P ≈ 0 in WG
Estimated Supply Temperature with perfect mixing
Case 5
70% Return & 30% Outside Air
Estimated supply discharge temperature
Conclusions • With the added vanes below the outside air damper, a perturbation
airfoil and mirrored rotation of one blender, this model shows that the minimum temperature across the cooling coil is calculated to be 42ºF.
• The average supply temperature discharge is calculated to be 46ºF whereas the theoretical perfect mixing case would be 49.24°F.
• An added bonus to this design shows that the mirrored blender enhances the temperature distribution on the cooling coils to be more symmetrical left to right.
Case 5
Case 6: Parameters • Star CCM+ software was used to model the Economizer.
• Given Values: – Air Flow CFM = 51,100 CFM – 20% Outside Air and Temperature = 0ºF – Supply Air Temperature Target = 55ºF
• Assumptions: – Return Air Temperature = 70ºF
Case 6: Parameters • Kept the Blender 5” from the floor • Kept the vanes below the outside air damper. • Kept the perturbation airfoil. • Kept the mirrored right blender (when looking down towards the
supply fan in the air handling unit.)
Custom Air Handler without Pre-Heating Geometric Side Elevation View
Pre-Filters
Cooling Coils 20% Outside Air Enters
Return Air Enters
Case 6
Blenders
Vanes
Perturbation Airfoil
Custom Air Handler without Pre-Heating Geometric Isometric View
Supply Fan
Humidifier Spray Nozzles
Blenders Cooling Coils
20 % Outside Air Enters
Return Air Enters
Case 6
Vanes
Pre-Filters Mirrored Blender
Perturbation Airfoil
Custom Air Handler without Pre-Heating Temperature Contour – Side Elevation View
Cooling Coils
Case 6
Custom Air Handler without Pre-Heating Cooling Coil Face Temperature – Front Elevation View
Cooling Coils (35º - max value)
Case 6
Bottom Coil Middle Coil Top Coil
Case 6
Custom Air Handler without Pre-Heating Cooling Coil Face Temperatures at Various Heights
Custom Air Handler without Pre-Heating Relative Pressure Contour – Side Elevation View
∆P =0.22 in WG
∆P =0.40in WG
Case 6
∆P =0.38 in WG
P ≈ 0 in WG
Estimated Supply Temperature with perfect mixing
Case 6
80% Return & 20% Outside Air
Estimated supply discharge temperature
Conclusions • With the added vanes below the outside air damper, a perturbation
airfoil and mirrored rotation of one blender, this model shows that the minimum temperature across the cooling coil is calculated to be 47ºF.
• The average supply temperature discharge is calculated to be 55ºF.
Case 6
Case 7: Parameters • Star CCM+ software was used to model the Economizer.
• Given Values: – Air Flow CFM = 51,100 CFM – 50% Outside Air and Temperature = 35ºF – Supply Air Temperature Target = 55ºF
• Assumptions: – Return Air Temperature = 70ºF
Case 7: Parameters • Kept the Blender 5” from the floor • Kept the vanes below the outside air damper. • Kept the perturbation airfoil. • Kept the mirrored right blender (when looking down towards the
supply fan in the air handling unit.)
Custom Air Handler without Pre-Heating Geometric Side Elevation View
Pre-Filters
Cooling Coils 50% Outside Air Enters @ 35ºF
Return Air Enters
Case 7
Blenders (5” above floor)
Vanes
Perturbation Airfoil
Custom Air Handler without Pre-Heating Geometric Isometric View
Supply Fan
Humidifier Spray Nozzles
Blenders Cooling Coils
50 % Outside Air Enters @ 35ºF
Return Air Enters
Case 7
Vanes
Pre-Filters Mirrored Blender
Perturbation Airfoil
Custom Air Handler without Pre-Heating Temperature Contour – Side Elevation View
Cooling Coils
Case 7
Custom Air Handler without Pre-Heating Cooling Coil Face Temperature – Front Elevation View
Cooling Coils (35º - max value)
Case 7
Bottom Coil Middle Coil Top Coil
Case 7
Custom Air Handler without Pre-Heating Cooling Coil Face Temperatures at Various Heights
Estimated Supply Temperature with perfect mixing
Case 7
50% Return & 50% Outside Air
Estimated supply discharge temperature
Conclusions • With the added vanes below the outside air damper, a perturbation
airfoil and mirrored rotation of one blender, this model shows that the minimum temperature across the cooling coil is calculated to be 52ºF.
• The average supply temperature discharge is calculated to be 52ºF.
Case 7
Summary • Four air handler design variations were evaluated for their resistance to
coil freezing. • The proportion of outside air was also evaluated at 20%, 30% and 50%
at two temperatures. • The final design variation shows very good resistance to freezing down
to temperatures of 0 °F (-18°C) and below. • The new design was implemented on all but one air handler at the
facility. • Last winter’s operation saw no freeze-stat alerts for the retrofitted
equipment, but several shutdowns for the unaltered unit.
