The Effect of Supply Air Ductwork Heat (or Moisture) Gain or Loss on Variable Air Volume Systems
Rushikesh Jog
HVAC Articles Writer For Thermal Control Business Update (HVAC Magazine) and OEM Update | HVAC Design Engineer & Business Development at Proficient | Post Graduate Diploma In HVAC
This work is licensed under Creative Commons Attribution-ShareAlike 4.0 International
This article was republished in the September Issue of the HVAC magazine "Thermal Control Business Update" under the title "Duct conditions outcome on supply airflow rate and efficiency". Here's the link:
In this article, I am going to speak about the HVAC system sensible heat (SH) or latent heat (LH) gain or loss due to supply air (SA) ducting and its effect on the working of variable air volume (VAV) systems.
There are 3 ways by which system heat gain or loss due to SA ducting occurs: conduction, air leakage & air infiltration in the ductwork. This results in a change in the volumetric flow rate of SA.
A)?The volumetric flow rate of SA is affected by duct heat gain or loss by conduction (assuming zero air leakage and infiltration) under 2 conditions:
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1.?The SA dry bulb (DB) is lower than the inside design DB.
If the SA ductwork is passing through the space/spaces in which it intends to supply air, the SH gain (supply air DB gain) by conduction will not result in a change in the room sensible heat (RSH). Although, if the supply duct is passing through an unconditioned/air-conditioned space/spaces which have DB higher than the supply air DB, then the duct will gain SH from the warmer air of that space/spaces. This will increase the RSH due to which the cooling capacity of the conditioned air is decreased. To compensate for this, the cooling capacity of the SA must be increased. Let’s take the example of a VAV system in which we try to maintain constant supply air DB, but the volumetric flow rate of SA is variable. In this case, the inside design DB is also kept constant. Assume that due to lack of or improper maintenance, the insulation keeps getting worn out. This will increase the heat transfer by conduction, resulting in duct heat gain, which in turn will increase the RSH.
RSH = SHC x V? x ΔT = SHC x V? x (DB1 – DB2), where
SHC = Sensible heat constant = 1.08 BTU. min/hr. ft3. °F
V? = Volumetric flow rate of SA (cfm)
DB1 & DB2 = DB of room air & SA respectively.
SHC, DB1 and DB2 are constant, due to ΔT is constant. Hence when RSH increases, V? increases as well.
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2.?The supply air DB is higher than the inside design DB.?
If the SA ductwork is passing through the space/spaces in which it intends to supply air, the SH loss from the ducting by conduction will not result in a change in the RSH. However, if the supply duct is passing through the space/spaces which have DB lower than the supply air DB, then this will decrease the supply air DB. This will result in a decrease in the heating capacity of the conditioned air. To compensate for this, the heating capacity of the SA must be increased by increasing the volumetric flow rate of SA. Now in this case, the RSH of the room still increases although with some delay; since the supply air DB is still higher than the inside design DB, albeit lower than intended. In this scenario, while using a VAV system, there is duct heat loss since the insulation is getting worn out. This in turn will increase the RSH, with some delay. In this case, let the RSH remain constant.
RSH = SHC x V? x ΔT = SHC x V? x (DB2 – DB1)
SHC, RSH and DB1 are constant. Hence, when DB2 decreases, ΔT decreases due to which V? increases.
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Note: In the first scenario, I considered the RSH to be increasing since this is opposite to intended cooling of the space under consideration. However, in the second scenario, I considered the RSH to be constant since RSH was going to increase nevertheless due to intended heating of the space under consideration.
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B)?The volumetric flow rate of SA is affected by supply duct air leakage (assuming zero conduction and air infiltration) under 4 conditions:
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1.?The supply air DB is lower than the inside design DB.
If the SA ductwork is passing through the space/spaces in which it intends to supply air, the loss of cooler supply air by air leakage will not result in a change in the RSH. But, if the supply duct is passing through the space/spaces which have DB temperatures higher than the supply air DB, then air leakage from the supply ductwork will result in a loss of SH from the ducting, though the supply air DB will remain constant. This will lead to a decrease in the cooling & ventilation capacity of the conditioned air. To compensate for this, the cooling & ventilation capacity of the SA must be increased. In this scenario for a VAV system in which we try to maintain constant supply air DB, the volumetric flow rate of SA is variable. In this case, the room design DB is also kept constant. Assume that due to accidental holes or through open spaces between poorly connected sections of the supply ductwork, there is some air leakage. This will reduce the cooling & ventilation capacity of the conditioned air, which in turn will increase the RSH.
RSH = SHC x V? x ΔT = SHC x V? x (DB1 – DB2)
SHC, DB1 and DB2 are constant, due to which ΔT is constant. Hence when RSH increases, V? increases as well.
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2.?The supply air DB is higher than the inside design DB.
If the SA ductwork is passing through the space/spaces in which it intends to supply air, the loss of warmer supply air by air leakage will not result in a change in the RSH. However, if the supply duct is passing through the space/spaces which have DB temperatures lower than the supply air DB, then there will a loss of SH from the ductwork, though the supply air DB remains constant. The RSH of the room still increases although with some delay since the heating capacity of the SA has decreased; since the supply air DB is still higher than the inside design DB, albeit lower than intended. The ventilation capacity also reduces, which also must be compensated. In this case, while using a VAV system in which we try to maintain constant supply air DB, the volumetric flow rate of SA is variable. In this case, the room design DB is also kept constant. Now due to air leakage, there is going to be a reduction in the heating & ventilation capacity of the conditioned air, although there will still be an increase in the RSH of the intended room.
RSH = SHC x V? x ΔT = SHC x V? x (DB2 – DB1)
SHC, DB1 and DB2 are constant, due to which ΔT is constant. Hence when RSH increases, V? increases as well.
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3.?The SA humidity ratio (w) is lower than the inside design w.
If the SA ductwork is passing through the space/spaces in which it intends to supply air, the loss of humid supply air due to air leakage will not result in a change in the room latent heat (RLH). But, if the supply duct is passing through the space/spaces which have w higher than the supply air w, then there will be a loss of LH from the ductwork due to air leakage, although the supply air w will remain constant. This will reduce the dehumidification & ventilation capacity of the conditioned air, due to which the RLH increases. To compensate for this, the dehumidifying & ventilation capacity of the SA must be increased. In this scenario while using a VAV system in which we try to maintain constant supply air w, but the volumetric flow rate of SA variable. In this case, the room design w is also kept constant. Now due to air leakage, there will be a reduction in the dehumidifying & ventilation capacity of the conditioned air, which in turn will increase the RLH.
RLH = LHC x V? x Δw = LHC x V? x (w1 – w2), where
LHC = Latent heat constant = 0.68 BTU. min. lb./hr. ft3. gr.
w1 and w2 = Humidity ratio of room air and SA respectively
LHC, w1, w2 are constant, due to which Δw is constant. Hence when RLH increases, V? increases as well.
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4.?The supply air w is higher than the inside design w.?
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If the SA ductwork is passing through the space/spaces in which it intends to supply air, the loss of dry SA due to air leakage will not result in a change in the RLH. But, if the supply duct is passing through the space/spaces which have w higher than the supply air w, then there will be a loss of LH from the ducting due to air leakage. This will still increase the RLH, although with some delay; due to which the humidification & ventilation capacity of the conditioned air is decreased. To compensate for this, the humidifying & ventilation capacity of the SA must be increased. In this scenario while using a VAV system in which we try to maintain constant supply air w, but the volumetric flow rate of SA is variable. In this case, the room design w is also kept constant. Now due to air leakage, there will be a reduction in the humidification & ventilation capacity of the conditioned air. This in turn will increase the RLH, albeit with delay.
RLH = LHC x V? x Δw = LHC x V? x (w2 – w1)?
LHC, w1, w2 are constant, due to which Δw is constant. Hence when RLH increases, V? increases as well.
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C)?The volumetric flow rate of SA is affected by supply duct air infiltration (assuming zero conduction and air leakage) under 4 conditions:
?
1.?The supply air DB is lower than the inside design DB.
If the SA ductwork is passing through the space/spaces in which it intends to supply air, the SH gain due to air infiltration will not result in a change in the RSH. However, if the supply duct is passing through the space/spaces which have DB temperatures higher than the supply air DB, then the supply air DB will increase due to the infiltration of warmer air. This will increase the RSH due to which the cooling capacity of the conditioned air is decreased. To compensate for this, the cooling capacity of the SA must be increased. In this scenario while using a VAV system, the volumetric flow rate of SA is variable. In this case, the room design DB is kept constant. Assume that due to accidental holes or through open spaces between poorly connected sections of the supply ductwork, there is some air infiltration. This will increase the supply air DB, which will reduce the cooling capacity of the conditioned air, which in turn will increase the RSH.
RSH = SHC x V? x ΔT = SHC x V? x (DB1 – DB2)
SHC and DB1 are constant. DB2 increases, due to which ΔT decreases. Hence when RSH increases, V? increases as well.
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2.?The supply air DB is higher than the inside design DB.
If the SA ductwork is passing through the space/spaces in which it intends to supply air, the ducting will gain SH due to air infiltration will not result in a change in the RSH. But, if the supply duct is passing through the space/spaces which have DB temperatures lower than the supply air DB, then the supply air DB will decrease. The RSH of the room still increases, although with some delay since the heating capacity of the SA has decreased; since the supply air DB is still higher than the inside design DB, albeit lower than intended.
In this case, while using a VAV system, the volumetric flow rate of SA is variable. The room design DB is kept constant. Now due to air infiltration, there is going to be a reduction in the heating capacity of the conditioned air, although there will be an increase in the RSH of the intended room with some delay.
RSH = SHC x V? x ΔT = SHC x V? x (DB2 – DB1)
SHC and DB1 are constant. DB2 decreases due to which ΔT also decreases. Hence when RSH increases, V? increases as well.
?
3.?The supply air w is lower than the inside design w.
If the SA ductwork is passing through the space/spaces in which it intends to supply air, the LH gained by supply duct air infiltration will not result in a change in RLH. However, if the supply duct is passing through the space/spaces which have w higher than the supply air w, then the supply air w will increase due to infiltration of humid air. This will increase the RLH due to which the dehumidification capacity of the conditioned air is decreased. To compensate for this, the dehumidifying capacity of the SA must be increased. In this scenario while using a VAV system, the volumetric flow rate of SA is variable. In this case, the room design w is kept constant. Now due to air infiltration, there will be a reduction in the dehumidifying & ventilation capacity of the conditioned air, which in turn will increase the RLH.
RLH = LHC x V? x Δw = LHC x V? x (w1 – w2)
LHC and w1 are constant. w2 increases due to which Δw decreases. Hence when RLH increases, V? increases as well.
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4.?The supply air w is higher than the inside design w.?
If the SA ductwork is passing through the space/spaces in which it intends to supply air, the LH gained by the duct due to air infiltration which will not result in a change in the RLH. However, if the supply duct is passing through the space/spaces which have w higher than the supply air w, then the supply air w will be reduced because of the infiltration of drier air. This will still increase the RLH, although with some delay; due to which the humidification capacity of the conditioned air is decreased. To compensate for this, the humidifying capacity of the SA must be increased. In this scenario while using a VAV system, the volumetric flow rate of SA is variable. In this case, the inside design w is kept constant. Now due to air infiltration, there will be a reduction in the humidification & ventilation capacity of the conditioned air. This in turn will increase the RLH, albeit with delay.
RLH = LHC x V? x Δw = LHC x V? x (w2 – w1)?
LHC and w1 are constant. w2 decreases due to which Δw also decreases. Hence when RLH increases, V? increases as well.
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References:
4. Cover Image of this article is a photo by?Ahmed Ibrahim?on? Unsplash
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This article was republished in the September Issue of the HVAC magazine "Thermal Control Business Update" under the title "Duct conditions outcome on supply airflow rate and efficiency". Here's the link:
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This work is licensed under Creative Commons Attribution-ShareAlike 4.0 International ?
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