[TRNSYS-users] too-low LWT for Type753 heating coil

[David BRADLEY]

Ajit,
   A very big temperature drop on the liquid side would suggest that 
there is only a tiny bit of liquid actually flowing through the coil. 
Type753 (and Type508) have an input that allows you to set how much of 
the air passing through the coil comes to equilibrium with the average 
coil temperature and how much passes through the coil unaffected. It may 
be that if you are supplying very hot water and you have a low bypass 
fraction input (in other words a lot of the air is assumed to come to to 
equilibrium with the very hot coil) then the coil would need to only 
trickle a little liquid through in order to bring the air up to 
temperature. I'd suggest looking at the air bypass fraction assumption a 
bit.
Best,
  David


On 1/2/2014 16:51, Ajit Naik wrote:
>
> Hello All + Happy New Year!
>
> I am modeling a liquid-source heating coil with known liquid inlet 
> temp that conditions outside air entering with a constant flow rate to 
> a constant desired outlet air temperature. My intention is to use the 
> bypass fraction scheme to select a liquid-side pump by recording the 
> coil liquid flow rate (fraction of max liquid flowrate that is NOT 
> bypassed and passed thru coil to condition airstream). This is 
> performed using a Type753e heating coil, a Type2 aquastat (heating), a 
> Type 15 weather data, and an equation block:
>
> 1.The EWT for the liquid loop (60C) and desired outlet air temp are 
> constant (20C) and directly specified as fixed inputs to Type753e.
>
> 2.Type2 observes the outside air temp from Type15 and generates "1" or 
> "0" depending on whether the outside air is above or below a specified 
> fixed setpoint including deadband (20 +/- 3 C).
>
> 3.This control signal is passed to an equation block where it is 
> multiplied by a maximum liquid flow-rate that is passed to Type753e as 
> the liquid inlet flow rate in order to shut off the liquid flow 
> entirely when heating is not required.
>
> 4.The not-bypassed coil liquid outlet rate required to provide the 
> necessary heating is recorded to get an idea of what sort of pump 
> capacity is necessary.
>
> The sim runs fine, but the liquid side temp drop is unreasonably low 
> -- something like 40C drop for a corresponding air temp rise of 3-4 C. 
> Correspondingly the not-bypassed coil liquid outlet rate is very low 
> as the temp drop of the liquid is so high.
>
> I have executed the exact same scheme to size a pump for a cooling 
> coil using the equivalent cooling coil component Type 508c... and this 
> works fine (reasonable rise in liquid temp and so reasonable 
> not-bypassed liquid flow whenever the coil is in operation).
>
> All inputs/params are in appropriate units and the max liquid inlet 
> rate is sufficiently high but still on a reasonable order of magnitude.
>
> I am aware that Type753e guesses an initial air outlet temperature 
> (equal to the initial guessed average coil temp) -- is it possible 
> that an excessively low guess value is the cause of the problem here?
>
> I appreciate any and all input!
>
> Best of luck in 2014.
>
> Ajit
>
> Ajit Naik
>
> Energy Engineer
>
> *dbHMS
> *303 W Erie Street, Suite 510
>
> Chicago, IL 60654
>
> p (312) 915-0557 x308
>
> f (312) 915-0558
>
> www.dbhms.com
>
>
>
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-- 
***************************
David BRADLEY
Principal
Thermal Energy Systems Specialists, LLC
22 North Carroll Street - suite 370
Madison, WI  53703 USA

P:+1.608.274.2577
F:+1.608.278.1475
d.bradley at tess-inc.com

http://www.tess-inc.com
http://www.trnsys.com

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