Duct conditions outcome on supply airflow rate and efficiency

Supply air (SA) is the conditioned air delivered to a space from an air handling unit. Rushikesh Jog, HVAC Design Engineer at Proficient, discusses the volumetric flow rate of SA, which gets impacted by various duct conditions, including heat gain or loss through conduction, air leakage, and air infiltration. These factors can alter sensible and latent heat in the room, affecting the efficiency of heating, cooling, and ventilation systems.

System heat gain or loss from supply air (SA) ducting can occur through three mechanisms: conduction, air leakage, and air infiltration within the ductwork. These factors lead to variations in the supply air’s volumetric flow rate.

The volumetric flow rate of SA is influenced by duct heat gain or loss by the conduction mechanism.

It occurs in the duct assuming zero air leakage and infiltration under two scenarios.

SA dry bulb (DB) is lower than inside design DB: If the SA ductwork passes through the spaces it intends to supply air, the SH gain (supply air DB gain) by conduction will not result in the room’s sensible heat (RSH) change. However, if the supply duct passes through unconditioned or air-conditioned spaces with DB higher than the supply air DB, then the duct will gain SH from the warmer air of that space/space. This will increase the RSH, which decreases the cooling capacity of the conditioned air. The cooling capacity of the SA should be increased to compensate for it. Let us take the example of a VAV system in which we try to maintain a constant supply of air DB, but the volumetric flow rate of SA is variable.

In this case, the inside design DB is also kept constant.

Assume that insulation is damaged due to lack of or improper maintenance. This will increase heat transfer by conduction, resulting in duct heat gain, which increases the RSH.

RSH = SHC x V̇ x ΔT = SHC x V̇ x (DB1 – DB2), where

SHC = Sensible heat constant = 1.08 BTU. Min/ hr. ft3. °F

V̇ = Volumetric flow rate of SA (cfm)

DB1 & DB2 = DB of room air & SA respectively.

SHC, DB1 and DB2 are constant due to ΔT being constant. Hence, when RSH increases, V̇ increases as well.

Supply air DB is higher than inside design DB: If the SA ductwork passes through the space/spaces it intends to supply air, the SH loss from the ducting by conduction will not change in the RSH. However, if the supply duct passes through the space/spaces with DB lower than the supply air DB, this will decrease the supply air DB. It will result in a decrease in the heating capacity of the conditioned air. So, the heating capacity of the SA must be increased by increasing the volumetric flow rate of the SA. Now, in this case, the RSH of the room still increases with delay. Since the supply air, DB is still higher than the inside design DB, albeit lower than intended.

In this scenario, duct heat is lost since the insulation is worn while using a VAV system. It will increase the RSH with some delay. In this case, let the RSH remain constant.

RSH = SHC x V̇ x ΔT = SHC x V̇ x (DB2 – DB1)

SHC, RSH and DB1 are constant. Hence, when DB2 decreases, ΔT decreases, and then V̇ increases.

Note: In the first scenario, I considered the RSH to be increasing since this is opposite to the intended cooling of the space under consideration. However, in the second scenario, RSH is constant since RSH increases due to the intended heating of the space under consideration.

Volumetric flow rate of SA affected by supply duct air leakage

Assuming zero conduction and air infiltration, it occurs under four conditions.

Supply air DB is lower than inside design DB: If the SA ductwork passes through the spaces in which it intends to supply air, the loss of cooler supply air by air leakage will not result in RSH change. But, if the supply duct passes through the space/spaces with 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. It will lead to a decrease in the cooling & ventilation capacity of the conditioned air. The cooling & ventilation capacity of the SA must be increased.

In this scenario for a VAV system in which we try to maintain a constant supply of air DB, the volumetric flow rate of SA is variable. In this case, the room design DB is also kept constant. Assume that there is some air leakage due to accidental holes or open spaces between poorly connected sections of the supply ductwork. It will reduce the cooling & ventilation capacity of the conditioned air, which will increase the RSH.

RSH = SHC x V̇ x ΔT = SHC x V̇ x (DB1 – DB2)

SHC, DB1 and DB2 are constant, so ΔT is constant. Hence, when RSH increases, V̇ increases as well.

Supply air DB is higher than inside design DB: If the SA ductwork passes through the space/spaces it intends to supply air, the loss of warmer supply air by air leakage will not result in RSH change. However, if the supply duct passes through the space/spaces with DB temperatures lower than the supply air DB, there will be a loss of SH from the ductwork, though the supply air DB remains constant. The RSH of the room increases with some delay since the heating capacity of the SA decreased. The supply air DB is still higher than the inside design DB, albeit lower than intended. The ventilation capacity also reduces, and it must be compensated.

In this case, while using a VAV system in which we try to maintain a constant supply of 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, the heating and ventilation capacity of the conditioned air is reduced, although the RSH of the intended room will still increase.

RSH = SHC x V̇ x ΔT = SHC x V̇ x (DB2 – DB1)

SHC, DB1 and DB2 are constant, so ΔT is constant. Hence, when RSH increases, V̇ increases as well.

SA humidity ratio (w) is lower than 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’s latent heat (RLH). But, if the supply duct passes through the space/spaces and w is 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. It will reduce the dehumidification & ventilation capacity of the conditioned air, due to which the RLH increases. The dehumidifying & ventilation capacity of the SA must be increased.

In this scenario, we use a VAV system to maintain a constant air supply with a variable volumetric flow rate of SA. In this case, the room design w is also kept constant. Now, due to air leakage, the dehumidifying and ventilation capacity of the conditioned air will be reduced, which will increase the RLH.

RLH = LHC x V̇ x Δw = LHC x V̇ 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

LHC, w1 and w2 are constant, due to which Δw is constant. Hence, when RLH increases, V̇ increases as well.

Supply air w is higher than inside design w: If the SA ductwork passes through the spaces in which it intends to supply air, the loss of dry SA due to air leakage will not change the RLH. But, if the supply duct passes through the spaces with w higher than the supply air w, then there will be a loss of LH from the ducting due to air leakage. It will still increase the RLH, although with some delay. The humidification and ventilation capacity of the conditioned air decreases. The humidifying and ventilation capacity of the SA must be increased.

In this scenario, we use a VAV system that tries to maintain a constant supply of air w. The volumetric flow rate of SA is variable. In this case, the room design w is also kept constant. Now, due to air leakage, the humidification and ventilation capacity of the conditioned air will be reduced. This will increase the RLH, albeit with delay.

RLH = LHC x V̇ x Δw = LHC x V̇ x (w2 – w1)

LHC, w1, and w2 are constant, due to which Δw is constant. Hence, when RLH increases, V̇ increases as well.

Volumetric flow rate of SA impacted by supply duct air infiltration

Assuming zero conduction and air leakage, it occurs under four conditions.

Supply air DB is lower than inside design DB: If the SA ductwork passes through the space/spaces in which it intends to supply air, the SH gain due to air infiltration will not change the RSH. However, if the supply duct passes through the space/spaces with 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, and the cooling capacity of the conditioned air will decrease. The cooling capacity of the SA must be increased.

In this scenario, the volumetric flow rate of SA is variable while using a VAV system. In this case, the room design DB is kept constant. Assume that there is some air infiltration due to accidental holes or open spaces between poorly connected sections of the supply ductwork. This will increase the supply of air DB, which will reduce the cooling capacity of the conditioned air, which in turn will increase the RSH.

RSH = SHC x V̇ x ΔT = SHC x V̇ x (DB1 – DB2)

SHC and DB1 are constant. DB2 increases, due to which ΔT decreases. Hence, when RSH increases, V̇ increases as well.

Supply air DB is higher than the inside design DB: If the SA ductwork passes through the space/spaces in which it intends to supply air, the ducting will gain SH due to air infiltration and will not change the RSH. If the supply duct passes through the spaces with DB temperatures lower than the supply air DB, the supply air DB will decrease. The RSH of the room still increases, although there is some delay since the heating capacity of the SA is reduced. 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 will 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 V̇ x ΔT = SHC x V̇ x (DB2 – DB1)

SHC and DB1 are constant. DB2 decreases, ΔT also decreases. Hence, if RSH increases, V̇ increases as well.

Supply air w is lower than inside design w: If the SA ductwork passes through the space/spaces in which it intends to supply air, the LH gained by supply duct air infiltration will not change RLH. However, if the supply duct passes through the spaces with w higher than the supply air w, then the supply air w will increase due to humid air infiltration. This will increase the RLH, and hence, the dehumidification capacity of the conditioned air decreases. 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, the dehumidifying and ventilation capacity of the conditioned air will be reduced, which will increase the RLH.

RLH = LHC x V̇ x Δw = LHC x V̇ x (w1 – w2)

LHC and w1 are constant—w2 increases and Δw decreases. Hence, when RLH increases, V̇ increases as well.

Supply air w is higher than inside design w: If the SA ductwork passes through the spaces in which it intends to supply air, the LH gained by the duct due to air infiltration will not change the RLH. However, if the supply duct passes through the space or spaces, w is higher than the supply air w. The w will be reduced because of the infiltration of drier air. It will still increase the RLH, although with some delay. The humidification capacity of the conditioned air is decreased. 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, the humidification and ventilation capacity of the conditioned air will be reduced. This will increase the RLH, albeit with delay.

RLH = LHC x V̇ x Δw = LHC x V̇ x (w2 – w1)

LHC and w1 are constant. w2 decreases, and so Δw also decreases. Hence, when RLH increases, V̇ increases as well.