Simple Question.....Simple Answer?

[Daddy Cool]

I was talking to someone i felt had better knowledge than me the other day (no, not my cat) and we had opposite views on the following scenario;

Chilled water coil, flowrate is constant, air across coil constant velocity and temp etc, but widen the delta T, and what happens to duty?

They couldn't substantiate their opinion more than 'because it does'

Just to clarify, i'm not talking about anything extreme, just move the 7°C flow and 11°C return to 6°C flow and 12°C return.

Simple???

[chemi-cool]

It might get too cold and than too hot.

There is a good reason for 7°C to 11°C.

[Daddy Cool]

Hi Chemi,

it's not so much of a controls issue, the central plant delivers 6/12, it was more to do with coil sizing. i have published data for the coil, but not at the 6/12 required. my question to them was could they re-calc the duty of the coil with a delta T of 6°K. the discussion surrounded what effect keeping all the variables of air volume and chilled water flowrate at a constant, but moving the delta T to a wider or narrower band would have on duty?

For instance, if the coil produced 5kW at Delta T of 4°K, would we gain or lose duty if the delta T were 6°K?

[Billy Ray]

Daddy Cool,

the coil would do more duty. There would be a greater temperature drop across the coil (air On & Off)

The effect would be a lower off coil condition.

Billy Ray

[frank]

It's quite simple really.

Use the basic formula and all becomes clear.

Q = m c dt

So, for a coil doing 5kW duty and a dt of 4k we can work out the mass flow rate

5 = ? x 4.19 x 4

Transposing, we get

? = 5/ (4.19 x 4) = 0.2984kg/s (assuming pure water)

So, with a mass flow rate of 0.2984kg/s and a dt of 6k we would then end up with the coil doing

Q = 0.2984 x 4.19 x 6

= 7.50kW

So the coil would then give you an additional 1.5kW or an increase of 20%.

[Daddy Cool]

Thanks chaps, Frank has written exactly what i said to them. My response was that surely it we widen the delta T, we have more duty, but they said, no no silly boy, you lose duty...that's why i asked how that would happen, and even said on that basis, if i have a flow of 6°C and water off at 6°C, the duty would be infinite???

i think i may have discovered where they went wrong.....software...i asked them to increase the delta T to suit my design, but i think it may have reduced the flow rate as a consequence.

Thanks for the back up boys, i was 99% sure i was right, but just needed to be sure.

[wambat]

In I-P
If BTU = GPM x (T1-T2) x 500 then 1 gpm x 6 *f x 500 = 3000Btu
If btu = 1 gpm x 4 *f x 500 = 2000 btu so the duty on the coil would increase.
Then if you say if I have a duty of 3000 btu and a delta t of 6 *f x 500 + 3000/6 x 500 = 3000/3000 =1 gpm.
If I have a duty 3000 btu and a delta t of 4 *F x 500 then 3000 btu/4x 500 = 2000 =3000btu/ 2000= 1.5 GPM so in order to get the increase I need more GPM
Just showing it in IP as Frank said in metric

[Daddy Cool]

The original question i asked the manufacturer was for the coil duty at the design spec, not 'standard sizing'

They advise as follows;

1250 CFM; 8 GPM with air on temps of 80/67(°F) with chilled entering at 42, leaving at 55, leaving air temp of 55°F making 35,416BTU sensible, and 53,781 total.

I wanted 6/12 flow and return .5l/s chilled flowrate, air on coil of 24/15.6°C at 590 l/s, but with the option of lifting airflow to around 640 l/s. i then needed the total and sensible.

They came back with some options, but the chilled flow and return were at 5.6°C/10.7°C & 12.2°C/15.3°C a max delta t of 5.1°K, so iasked them to increase the deltat t, which promted the 'why?' question.

i explained that i wanted maximum sensible and total for that sized coil, but they said, it you open the delat t wider you loose duty....,i politely said don't see how, and then the fight started.

i was left feeling i had missed something, so asked them to explain how this could be......

[wambat]

The original question i asked the manufacturer was for the coil duty at the design spec, not 'standard sizing'

They advise as follows;

1250 CFM; 8 GPM with air on temps of 80/67(°F) with chilled entering at 42, leaving at 55, leaving air temp of 55°F making 35,416BTU sensible, and 53,781 total.

I wanted 6/12 flow and return .5l/s chilled flowrate, air on coil of 24/15.6°C at 590 l/s, but with the option of lifting airflow to around 640 l/s. i then needed the total and sensible.

They came back with some options, but the chilled flow and return were at 5.6°C/10.7°C & 12.2°C/15.3°C a max delta t of 5.1°K, so iasked them to increase the deltat t, which promted the 'why?' question.

i explained that i wanted maximum sensible and total for that sized coil, but they said, it you open the delat t wider you loose duty....,i politely said don't see how, and then the fight started.

i was left feeling i had missed something, so asked them to explain how this could be......

This whole discussion is about misunderstanding for example> Yhe formula for a chill water heat exchancer is Q = GPM x delts "T" x 500 a constant

Now when Heat xchangers are mfg. they are made to transfer the maximum amount of heat at a set GPM at set Delta "T" Now if you try to change the delta "T" and try to maintain the max heat transfer the GPM has to increase. In other words any change in ither the flow rate or the delta "T" will change one or the other
So when the people you are talking with say you will loose duty they were right what they didn't say was unless you increase flow

[Daddy Cool]

i think you are right, a large part of the mis-understanding came from each party not fully understanding the others constants and variables!

i know that i can't change the chilled flow and return, they are from central plant and they won't be changed to suit me. i can to an extend control the flowrate, and the CFM across the coil. i'm not sure they really understood that's it's not a 'stand alone' project where i can control and amend other items. some things on this project are fixed and i have to find a solution to work around them. maybe i will try another trans-atlantic call to re-explain my needs. i feel they were using software that automatically changed the flowrates to suit the new Dt, but as a consequence it reduced the duty, hence their comments.

[Peter_1]

This is a linear function, so if DT increases from 4 K (11°C to 7 °C) to 6 K (12°C to 6°C) K and all the other variables remains the same, then your load increase in the same ratio from 4 to 6 or IOW, 50%.