2-pipe fan-coils CHW and HW gpm 5

[ClimateFrontier]

For 2-pipe fan-coils, does design CHW gpm have to equal design HW gpm, or can HW gpm be smaller? ie during cooling vfds on pumps would push more water through fully open valve than during heating.

Thanks!

 

[DRWeig]

CHW gpm does not have to equal HW gpm, and it would be very rare to find them to be equal. Depends on ratio of cooling design load to heating design load.

If they're equal, HW flow will be smaller flow since approach (difference between supply water temperature and supply air temperature) is much larger with heating, and also the cooling load is part latent while the heating is all sensible. So only if design heating load is much higher than design cooling load will HW flow be larger than design cooling flow.

Down south here (Georgia, Tennessee), CHW flow is always much higher at design with typical HW supply design temp of 160°F to 180°F and CHW supply temp of 40°F to 45°F, cooling supply air temp of 55°F and heating supply air temp of 100°F to 120°F.



Good on ya,

Goober Dave

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[cdxx139]

It doesnt have to, but wouldnt it be nice if it did?

What if you get the CHW gpm, then back engineer the HW gpm to match. This would require the HW temps to be reduced, and assuming you have condensing boilers, that is a big plus to your system efficiency.

It really depends on your valving at the FCU. Assume modulating valve with automatic flow limiting valve?

knowledge is power

 

[ClimateFrontier]

Yes the valve is modulating with automatic flow limiting valve. Thanks for your answer.

 

[Drazen]

Well, the valve can be three-way valve, but to make sure they work as modulating, you need to check controls, are they 3-point or really modulating, which is not in frequent use in fan coils.

Reason is obvious - as you have large difference in needed gpm for cooling and heating (in most of cases, as colleagues mentioned), it is almost certain that your modulating valve will not have adequate hydraulic authority either in heating or in cooling mode.

Which means you will have disfunctional controls in one of these modes.

 

[DRWeig]

Drazen makes a really good point. In fact, I see many, many systems with proportional + integral control of the valve during cooling mode, but two-position control with a fairly narrow deadband in heating mode (again, I'm in a warm and humid climate, so heating water flows are very low compared to cooling water flows).

During heating season, the valve allows full design flow just a little bit after it starts to open, so why bother with trying to control it accurately? If the boiler plant control is set up with hot water temperature reset, decent space temperature control can still be maintained. Just not as tight as in cooling mode. I don't hear a lot of complaints from the occupants except in zones with vastly oversized coils.

Good on ya,

Goober Dave

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[ClimateFrontier]

The coil has 4 rows, so would it be a good idea to have hot water reset from the 180 F EWT through a mixing valve, (if hot water reset is not available on the boiler)? Thanks for your responses.

 

[Drazen]

i believe how water reset is always good thing, if you have engineering means to calculate to make sure that outlet air temperature of fan coils will not fall beyond 35 degree celsius (that is 95F, some recommondations say 30 deg. C - 86F - is enough to avoid cold draft feeling, but i personally do not believe that).

as ncpe mentioned, you can back-engineer if you have available means.

some very esteemed manufacturers of fan-coils left us with manual tables where such back-calculation is very tedious, while daikin, who is rather new to hydronic systems, has handy small software which allows you to back-engineer heating capacity with flow determined for cooling.

with or without this small software, such a task is rather tedious, can save very decent amount of money to client, and i would never do it without extra charging my engineering work (if that is not design job already contracted for some arabian emir with "all special requests included" clause).

 

[cdxx139]

My expericence of back engineering was not so difficult. Say the CHW gpm flow was 4.2 and the HW gpm flow was 1.4 at 180/160. I selected the FCU heating with a fixed gpm of 4.2 (same as CHW), then reduced the HW temps, so they matched the load at the CHW flow of 4.2 gpm.

This worked great for the FCU's, as I got to something like 95 EWT, which was excellent. But when I reviewed the other heating required on the project, specifically the MUA unit for the laundry room, I needed 110 EWT to meet the load.

So do the calucalation, check all the equipment on the project, and set the HW temps at what is needed to meet the load of all units.

Good luck

knowledge is power

 

[317069]

Climate
- for your main question about if the CHW gmp equal to HW gpm, your load calculation will tell you how much gpm do you need for your application, in addition to the fan coil product data will tell you too about unit capacity and requirments in cooling and heating mode.

Drazen
- hot water reset is good thing but where, on boilers or fancoils?

 

[Drazen]

cdxx139,

naturally, our feel of what is more or less tedious does not have to be exactly the same. after long years i grasped rather firm feeling what is referent level of designer's engagement, and what goes beyond that, but that may differ in different parts of the world.
i did not understand you completely. i was talking about need to check air leaving temperature of FCU, but your are mentioning water reset temperature. if that is your water reset temperature, than air leaving temperature will go dangerously close to cold draft-feel temperature and the problem is exaggerated by the fact that all FCU's are oversized for heating mode, means they will reduce mean water temperature over surface very much. of course, heat transfer is not so simple process, and overall mean temperature will be reduced that way somewhat reducing heat transfer effectiveness, but if you are in position to use any simulation software, you can find that your air leaving temperature will come very close or bellow "danger threshold" for cold draft feeling.

and this problem is worst during lowest-load condition, being additionally emphasized in interior areas of commercial building that may have large cooling needs, but low heating needs.
my belief is that this problem is far from being simple, but, of course, we may have different opinions.

317069,
my understanding was that we are talking about central supply temperature reset, modulating on FCU level is mentioned in my previous post.

 

[317069]

Drazen
my understandin (please correct me if I am wrong)is that the hot water reset is to save energy by modulating the water temperature in the boiler depend on the outside temperature, in other word to control the boiler and reduce fuel consumption, now if we use this control in 2 pipes fan coil provided already by 180 F hot water what the idea behinde reducing this temperature by mixing it with 160 return water. also what about the space thermostat?

 

[ClimateFrontier]

If hot water reset is not available for the boiler, then the idea was to mix the return water with the 180 F entering water. However, if we mix the return water how do we deal with the low pressure of the return water in comparison to the high pressure of the 180 F entering water? Thanks, appreciate your thoughts.

 

[cry22]

Hot water reset is required by code (ASHRAE 90.1)
using 180F at part load, you will require no more than 0.2 or 0.3 GPM through you heating coil.
1. your valve will not go down that low, it will get may be down 0.5 GPM, if that, meaning you will be overheating teh space.
2. you will be in laminar flow and start building air in your system. the water velocity would be so low (under 1.5 FPS) that the air will not be entrained back to the air separator.

With HW reset, your valves will always be almost fully open, which is good.
And yes, your pump VFD will be kind of useless in winter but your system will work. Your savings are at the boiler.

 

[ClimateFrontier]

With the boiler reset control, can it be used to limit the water temperature to no more than 150F at any time, even on cold days? Thanks!

 

[317069]

ASHRAE 90.1 Standard asked for hot/cold water reset to improve the effeciency of the primary equipments( chiller/boiler)in 300000 BTU/hr systems or more, it has provided three control aproches but didn't say how to do. I mean didn't say if you have to control boiler burners forexample or mix the water.
also ASHRE suggest to use variable speed pumps and advise to not mix between hot water reset and variable pumping.
it is all about the coil performance and capacity at different conditions. but any way the control work fine down to 50% of the full load, so don't expect to control a system from 0% to 100%