HVAC reheat coil selection priorities

[ACD1]

I am trying to become better at hvac coil selection. I've recently run into situations where compromises have to be made, and I'm unsure of what my priorities should be in the final selection.

I was trying to select a heating coil for an FCU with approximately 4,000 cfm, EAT=55, LAT=85. I had 180 degree water, and wanted to design to a 20 degree water delta T. With LWT=160 and letting the LAT float yielded 95 degrees, but required 18gpm, 2.7fps tube velocity. Letting the LWT float yielded a LWT of 135 degrees, 6 gpm, 85 degree LAT, but 0.9fps tube velocity and a reynolds in transitional approximately 8500. It's the same coil either way, but I'm trying to figure out what to schedule for the control valve. It seems like such a wide range between 6 and 18 gpm. What's the best information to schedule and why?

I was also trying to select a VAV reheat coil - 1,200 cfm, 85 degree LAT. Price had 3 options, a 1-row standard, a 1-row high capacity, and a 2-row coil. I was again using EWT=180 degrees, looking for LWT=160 degrees. I got the following: 1) 12" inlet, 2-row coil, LWT=117, LAT=85, 1.26 gpm, WPD=0.42'. 2) 12" inlet, 2-row coil, LWT=160, LAT=104.5, 6.7gpm, WPD=8.2'. 3)12" inlet, 1-row HC coil, LWT=160, LAT=85, WPD=13.5'. 4)12" inlet, 1-row standard coil, LWT=160, LAT=79.1, WPD=1.19'. My instinct here says to go with the 2-row, because of not reaching the proper temperature - but even 8' of WPD is too high.

Any insights as to priorities in these situations and what to do when maybe your coil options are limited would be appreciated.

 

[EnergyProfessional]

You have to select a coil large enough. since there are only 3 options, it will be a bit "too large". the discharge air temperature sensor will measure DAT and modulate the control valve to the desired temperature.

 

[ACD1]

I guess the real question is, all other performance parameters being acceptable, what situation is preferable for scheduling your control valve and coil: 1) That the LWT is what you want (in my case LWT=160) - but the LAT is beyond what I need, or 2) That the LWT is much lower than desired, but the LAT is what I desire (in my case 85 degrees)?

 

[EnergyProfessional]

if LWT is lower, what is the damage? None i suppose.
DAT is what is needed to meet the heating load.

 

[ACD1]

With such a low LWT, aren't you "choking it off?" Different texts warn you about thermal shock to the boiler, laminar flow through the coil, as well as coil capacity drop off. But if the overall system is designed to a 20 degree water delta T, and relatively few instances where you're greatly exceeding the 20 degree delta target - I suppose you're probably all right? In the first case I noted for the FCU, doesn't it follow that there would never be a time when 18gpm would ever be used, and - since there are no other coil choices and no way to decrease the EWT, I'd be using < 6gpm no matter what? For the FCU, I was leaning towards the 6 gpm option. For the VAV reheat coil, I was similarly leaning towards the 1.26gpm option - but the LWT is 117, very far from 160 degrees.

 

[EnergyProfessional]

energy extracted is the same regardless of dT on water side (at a given air temperature rise). only gpm changes.
modern design is for higher dT on water side (30°F) and lower water temps on supply side to sue condensing boilers.

Look at thermodynamics book, energy balances.

 

[ACD1]

I appreciate your feedback, EnergyProfessional. I understand the energy total standpoint, but are you (or is anyone else) aware of a situation where there were temperature control issues after using a larger-than-required reheat coil? Why all the caution regarding laminar flow in the coil?

 

[EnergyProfessional]

less heat transfer between fluid and wall when flow is laminar. However, the limiting factor is not heat transfer between water and tubing, it is the transfer between tube/fins and air.
Coils usually gets sized by some manufacturer software and this will tell you when flow is too low, or high.

since we have equipment in incremental sizes, and conditions (e.g. incoming air temperature) change, the coil always will be a bit "too large". this is made up by valve modulation. Like a car, they don't sell you a car with an engine for every speed you want to drive, they give everyone the same engine and a throttle.