Coil capacities - rule of tumb

[remp]

I have an old air handling unit with a heating and cooling coil.
I need to get an idea of thier capacity but have no technical info on them. Is there any rule of tumb I can use based on the corss sectional area, no. of rows, fins/m2, tube dia ect... to get an idea of their capacity?

 

[AbbyNormal]

measure the water flow/temps if hydronic,measure the coil performance

count the rows, fins, tube size and reverse engineer using a coil program like say from USA Coil & Air or get an old Bohn catalog



Take the "V" out of HVAC and you are left with a HAC(k) job.

 

[gepman]

This thread, thread403-201945, had an interesting paper on coil performance calculations.

 

[CountOlaf]

You may be looking for something more exact and the following may be known to you already, but I'll mention it anyway since I frequently use the following on existing systems just to get me in the ballpark (e.g., project proposal phase or something where I'd just like to quickly get an idea of how big of a system we have):

1. Airside estimate: q (sensible tons) = [1.1 * CFM * delta T] / 12,000 BTUH/ton.

Try a typical airside delta T of say 25 deg F (assume 80 deg F return air temp - 55 deg F supply air temp) unless you have better info as to the actual entering & leaving temp's. Regarding CFM if you know the supply fan's max capacity, use that. If not, you know the face area of the coil (SF) and you also know that the typical maximum design velocity across a cooling coil is usually 500 to 600 FPM. Therefore you can determine a likely airflow rate. Now just plug and chug for sensible cooling capacity.

If you happen to know the dry and wet bulb conditions of the entering/leaving air (or feel like quessing at them as well) you can get the enthalpy points instead and calculate the total coil capacity (sensible plus latent) using the equation:

q (total tons) = [4.5 * CFM * delta h] / 12,000 BTUH/ton

2. Waterside estimate: q (tons) = [500 * gpm * delta T] / 12,000 BTUH/ton.

Look at the pipe size serving the cooling coil. Say it's a 2". If you were designing a new system, you'd want to have a maximum gpm through a 2" pipe of around 50 gpm (moderate velocity of ~ 4 fps and pressure drop of 4'/100'). Use a typical waterside delta T of say 10 deg F (assume 55 deg F return water temp - 45 deg F supply water temp) unless again, you have better info as to the actual water supply and return temperatures (it's possible 12 deg F delta T is more likely at your location for instance). Now just plug and chug again for a "waterside" comparison. In my example, the coil would be about a 20-ton.