air / water heat transfer
air / water heat transfer
(OP)
i have a question about delta t for heating coils in fan coil units
it seems like a stupid question but anyway
if a heating system is designed as 82deg c flow and 71 deg c return therefore an 11 deg c delta t. If all the coils in the system are also designed for 11 deg delta t at design flow rate of water. Assuming that heating water flows and airflows across coils are at design, what would the effect of running the boiler at 70 deg c be?
would the return water temp be 11 deg c lower? (i.e. 69 deg c)
would the air still achieve its design heat gain across the coil? assuming max air temp design is 24 deg c
basically what im wondering is how important is the flow temp of the water?
i know in design Q = mc delta t is used for boiler size, coil sizes, airflow and water flowrates but that only uses a delta t and not an actual temp.
it seems like a stupid question but anyway
if a heating system is designed as 82deg c flow and 71 deg c return therefore an 11 deg c delta t. If all the coils in the system are also designed for 11 deg delta t at design flow rate of water. Assuming that heating water flows and airflows across coils are at design, what would the effect of running the boiler at 70 deg c be?
would the return water temp be 11 deg c lower? (i.e. 69 deg c)
would the air still achieve its design heat gain across the coil? assuming max air temp design is 24 deg c
basically what im wondering is how important is the flow temp of the water?
i know in design Q = mc delta t is used for boiler size, coil sizes, airflow and water flowrates but that only uses a delta t and not an actual temp.





RE: air / water heat transfer
Your system appears to be designed to supply a certain amount of heat from 82°C water to 24°C air, i.e., a 58°C temperature delta. Reducing that delta to 46°C means less heat supplied, so the return cannot have dropped 11°C.
TTFN
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RE: air / water heat transfer
Which is often fine for most conditions. When it becomes not fine is at design OA conditions, when your reduced air temperature causes control valves to be fully open and eventually not enough flow to meet demand. If you think of it on the demand side, the building will just require one Q for the given OA condition regardless of what temperature the boiler provides. But if you want to reduce temp (fine for warmer weather) your flow (m) has to go up to keep the same Q...
Hopefully helpful? CB
RE: air / water heat transfer
if air design conditions are as follows (example only)
air flowrate = 0.3 m3/s
outside air = -2, design supply air = 22 (air DT = 24 deg c)
Heating requirement to do this 8.82kW
Therefore fan coil unit heating coil size is 8.82kW, heating system has 11 deg c DT (82 flow, 71 return). (which are the standard f & r temps for LTHW systems)
so water flowrate to do this = 0.191 l/s
assuming we are achieving these air and water flowrates and they are constant, and assuming it is -2deg c outside.
so at 82 deg c water flow we will have 22 deg c air.
what will happen when water flow temp from boiler is reduced to say 70 deg c?
what will air temp now be?
how to calculate this?
how to calculate new DT?
would it not be DT= q/mc....in that case 8.82/0.191*4.2= 11
this is what confuses me. has Q changed?
does Q stay constant because that is what the coil physically is?
Or is a coil actually specified/supplied for kW, and a DT only at design water flow temp (i.e. 82)
in that case Q of the coil changes with flow temp, and then DT will also change so flow will need to increase to keep DT at the desired 11.
could somebody clarify please!
RE: air / water heat transfer
Imagine if the entering water temperature were the same as the entering air temperature.
RE: air / water heat transfer
Q is proportional to dT. Reduce dT, you reduce Q.
TTFN
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RE: air / water heat transfer
so if water flow temp drops from 82 to 70 and both flowrates across coil remain the same and outside temp remains same
we know that Q and DT have both changed
How do i work out what they have changed to? as i cannot use Q=mcDT
is coil output directly proportional to flow temp of water?
i.e. 82deg c = 8.82 kW = 100%
so 70 deg c = 7.5kW = 85%
if it was then the new DT could be worked out with
DT = Q/MC
7.5/0.191*4.2 = 9.35
is this correct??
RE: air / water heat transfer
70-24 / 82-24 = 79.3%
TTFN
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RE: air / water heat transfer
in the equation you used above you got the ratio between a 70 deg water flow temp to heat air by 24 deg - to - an 80 deg water flow temp to heat air by 24 deg
this was 79.3%
so if the 100% output of the coil at 82 deg c was 8.82kW then with 70 deg c flow it is now 6.99kW
so new water DT = 6.99/0.191*4.2 = 8.72 deg c (was 11)
is this correct, and is this the only way to work out the new DT?
the reason why I am confused is because Q = mcDT does not allow me to work out new DT because all I know is M and C.
so is this ratio method the only way?
RE: air / water heat transfer
RE: air / water heat transfer
The water side can be written q=500gpmdeltaT
The air side can be written q(sensible heat)=1.08cfmdeltaT
Set both sides of equation equal to q or
500gpmdeltaT(Water)=1.08cfmdeltaT(Air)
Assuming gpm and cfm are constant,
Changing the deltaT(Water) will effect the deltaT(Air)
RE: air / water heat transfer
sailcat 753 - yes i know what you have said above but my query is how do i work out the new DT of the water if the flow temp has changed from the design temp, if water flowrate remains constant? see above example @ 18:31
RE: air / water heat transfer
If you need the same temperature for the air side, to get the capacity out of the water, you will likely require a different coil. Possibly more rows (or fins per inch), or a different construction.
So give a coil supplier your desired conditions, and they will give you the deltaT and other useful info.
RE: air / water heat transfer
RE: air / water heat transfer
RE: air / water heat transfer
TTFN
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RE: air / water heat transfer
Heres my version of the problem:
Nearly all manufacturers data on chilled water coils is for 6/12 degrees C water.
I am looking at the impact of using 9/15 chilled water in a datcentre to save energy (More hours free cooling and is above dew point so no re-humidification load) and need to know how much the recirc units will be de-rated when using the higher temperature.
Yes I know I should ask the manufacturer but thats no fun...
Given that the airflow over the coil, coil surface area, air temp, and max water flow rate in the coil are all identical, is there no way to calculate the reduction in heat transfer efficiency?
RE: air / water heat transfer
TTFN
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RE: air / water heat transfer
Although they asume that surface heat transfer cooeficients are constant, so I wouldnt use them where the air flow rate also changes.