Dual Duct Hot Deck Capacity
Dual Duct Hot Deck Capacity
(OP)
Is there a rule of thumb on the hot deck capacity of a dual duct air handler? By that I mean is there a way to estimate how much hot water I need in the hot deck coil?
I have an exising dual duct system with no labels on the unit, HW pump, heat exchanger, etc. I will be making some changes to the steam system serving the hot water heat exchanger, so I need to determine how much heat is needed. I don't know water flow or delta-T, so that route is out. I don't know Cv on the steam valve serving the heat exchanger, so that route is out.
All I know is total CFM of the dual duct air handler, which is approx 35,000 CFM. From the looks of the hot and cold deck, I would guess that 1/3 of the air passes through the heat coil and 2/3 through the chilled; but, this is just a guess.
Based on this limited information, is there a way to approximate the amount of heat I need for this system? Does a typical dual duct system have a constant hot deck discharge air temp that could help me? If so, what is a typical discharge temp?
I am specifically interested in the summer months, so heating outside air should not be an issue. Any ideas?
I have an exising dual duct system with no labels on the unit, HW pump, heat exchanger, etc. I will be making some changes to the steam system serving the hot water heat exchanger, so I need to determine how much heat is needed. I don't know water flow or delta-T, so that route is out. I don't know Cv on the steam valve serving the heat exchanger, so that route is out.
All I know is total CFM of the dual duct air handler, which is approx 35,000 CFM. From the looks of the hot and cold deck, I would guess that 1/3 of the air passes through the heat coil and 2/3 through the chilled; but, this is just a guess.
Based on this limited information, is there a way to approximate the amount of heat I need for this system? Does a typical dual duct system have a constant hot deck discharge air temp that could help me? If so, what is a typical discharge temp?
I am specifically interested in the summer months, so heating outside air should not be an issue. Any ideas?





RE: Dual Duct Hot Deck Capacity
It seems to me that if you take about 33% of CFM for heating or about 11,000 cfm and assuming a 20*F drop across the coil at the most heat needed then: BTU = CFM x temp diff x 1 = 11,000cfm x 20*f x 1 = 220,000 BTU'S and that would work out to~25 GPM at the required head.
RE: Dual Duct Hot Deck Capacity
In summer, hot OA can provide you the required heat.
As a rule of thumb, 70% of the total air should be heated from ? to ? (assume at least 70).
RE: Dual Duct Hot Deck Capacity
70% of air goes though heat coil? I was thinking only around 30%.
I reason I'm not concerned with winter is because the changes in the steam system will only be apparent in the summer. Winter operation will be the same as it is now, which is working fine.
RE: Dual Duct Hot Deck Capacity
Please note that system operation depends to the mixing box function, system configuration, and other factors.
A system I designed last year, would reset hot deck temperature in summer from 105 to 55 in order to provide the required "ventilation" air for the system. The ratio for coil selection was 90% cold deck 80% hot deck. One fan at 100%.
RE: Dual Duct Hot Deck Capacity