Dual Duct Hot Deck Capacity

[BronYrAur]

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?

 

[imok2]

"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" (quote) The hot deck is normally controlled by either a duel input control with OSA senser and hot deck sensor. This is a control that resets the hot deck as OSA goes up or down another control is called a Master control which measures OSA that resets a submaster which measures the hot deck. They both do the same function
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.

 

[hmm97]

I understand you are not concern about winter operation when there is ra eal need for heat. Sounds odd. Winter total load is block heating load plus heating the air from cold deck.
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).

 

[BronYrAur]

hmm97,

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.

 

[hmm97]

Sorry. I meant 30% Hot deck- 70% cold deck.
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%.

 

[BronYrAur]

Thanks, That gives me what I need for a maximum steam load in the summer. Appreciate the help!