boiler primary/secondary pumping 1

[swoosh172]

I am reviewing a design with a primary building hot water loop serving air handlers and vav's with a total hot water flow rate of 200 gpm on the design day. The equipment is designed for 180F entering water temperature and 160 leaving. The design has two condensing boilers that can run in parallel, each with a dedicated 30 gpm circulator. There is a 12 inch "no head loss" connection between the primary and secondary loops. My question is what temperature should the boiler supply water be on the design day? I figure above 200F but i'm not sure the boilers can do that high being that they are small modulating condensing boilers better served with lower water temperatures. 60gpm @ 200F mixing with 140gpm @ 160F equals ??.

 

[imok2]

Go here:

http://wea-inc.com/calc-frm.htm

 

[marcoh]

Boilers will need to supply water at 226F which is an issue.

Looks like the operating temperatures of the field heating equipment does not match the operating temperatures of the condensing boilers so you have an issue.

 

[swoosh172]

imok, that website is aweomse, thanks! I was afraid that these boilers could not handle the load, i'm not sure what to do. If the primary loop has a delta t of 20 degrees, what is a typical boiler loop delta t? 30 degrees? I am thinking the boilers are undersized and we need bigger boilers that would have bigger circulators to inject more hot water into the main loop.

 

[imok2]

Well if the mixing temp is 172*F the question is ; is that the worse case or will diversity be such that you would be needing less than 180*F in most cases ? I personally would go with bigger boilers after it's your butt on the line if it;s is too small.

 

[Drazen]

you did not mention actual capacities though I would not take for granted that primary and secondary are absolutely equal.

I admit to not understand how did you come to your required temperature at primary.

from my understanding, your boilers need to supply 180F, and that is it, all the rest is in capacities of boilers/consumers.

The fact that your boilers are condensing ones exactly points to the situation that they can handle high temperature differences as they can stand very low return temperatures, so as I said all is in capacities.

 

[Drazen]

Ops, I apologize, I completely messed primary and secondary flows (after full day of calculations).

On second thought, I would go with imok - reasonable assumption is that initial design clearly included diversity.

To "pull myself out" with one scenario: Is it two pipe or four pipe system? If it's two pipe, flow on secondary can be big because of colling flow needs and heating delta t is only formal, given by manufacturer while heating capicity is oversized as design was based on cooling capacity.

 

[317069]

- You said that you are revewing a design? did you talk with the designer?
- is this design still in design stage or has been built?
your load looks like equal to a 2000 MBH, devided between two boilers, I guess they are equal.
30 gpm circulator capacity for a 1000MBH boiler looks a small flow rate and to give a 20F delta, it needs 226F or 108C(above boiling temperature which is 100C), and that is an issue as marcho said above. 1000MBH boiler needs about 65 gpm circulator (for 25F delta)
- is this 30gpm circulator is a factory mounted circulator?
if "yes" you should contact the boiler manufuctrer and discuss with them, if some one other than factory has chossen this circulator then you should review that with him/her.

 

[swoosh172]

I spoke to the designer, his sequence of controls for the hot water pumps was for them to run in lead-lag fashion, and the pump schedule was for each pump to be rated at 100gpm, which is why I thought that total hot water flow was 200gpm. He told me that each pump is rated for the full design flow, but they are controlled in parallel operation for some reason. I thought parallel pumps should be designed for half the total flow? he didn't have a good reason for why they were both sized for 100% flow. The boilers are 450MBH each, and the circulator was chosen by the boiler manufacturer.

 

[cry22]

1. condensing boilers cannot deliver 200F water - Condensing boilers are designed for 140F water typically, not 180 or 200. Some cannot provide wtr above 140. OK some claim they can do 180, but never heard of condensing boilers delivering 200 wtr

2. Language of circulators are used for cast iron boilers to keep return water to the boiler above 140F - not for condensing boilers for which the lower the wtr temp, the better.

3. primary pumps should be sized each for one boiler total flow, secondary pumps are corectly sized for 100% of total flow, they need VFD's though and even PRV at discharge.

4. DO NOT Oversize Boiler too much, especially Fulton boilers, they have a maximum No of cycles at 12,000 per year, if too much oversized, they will cycle too much and you will wack your 10-year warranty in one winter. another reason not to operate BOTH boilers together with Fulton. Nothing against Fulton, just make sure your design matches their warranty.

5. Smoosh - Parallel pumps should not be designed for half the flow because the system curve is at a different point. Google parallel pumping. 2 pumps at 100% in parallel do not deliver more than one pump plus about 10% to 15% (that's right, check it out).

6. What is this business of the "circulators are sized by the boiler manufacturer"? Totally unacceptable in my book. Pumps must be sized by the Engineer, not the boiler MFTR. DO NOT let the MFTR design or size for you. It is a recipe for disaster, MFTR Reps are salesmen, Not Engineers (even those that hold engineering degrees).

 

[willard3]

I expect that this board is substituting for professional input for someone who doesn't do Engineering. You will need more than this board to review a design competently and using this board will not substitute for professional help.

 

[cry22]

Hey willard, where's the Looooove, man ))

 

[317069]

Thank you for this useful information.
There is a little difference in English between "circulator sized by manufucturer" and "factory mounted circulator".
I guess that the factories have a lot of engineering experience and expertise, more than any individual degree holder.