Primary/Secondary Piping Arrangement

[supaman79]

Guys,

This might be a simple physics question, but here it goes. On a primary/secondary piping system, physically how big does the primary loop have to be. What pops out at me right away is that, I would assume that you would need to have enough water in the primary loop to heat the secondary sources. Is there a rule of thumb, or do I need to apply some simple math to this. Can anyone help?

 

[imok2]

There is no rule of thumb that I'm aware of because each job is a different entity onto itself. The total required amount of water circulated depends on the total load applied to it. This will determine the primary (Plant)requirments. The secondary is the required load so your assumtion is correct.

 

[friartuck]

There are so many different ways to skin a cat.

I assume that you have a boiler system (or chiller) with a primary 'shunt circuit' feeding a loop. The primary then has a number of secondary offtakes.

I use a simple method which hasn't failed me yet.

A. Calculate your secondary requirements. i.e 3 sub-circuits at 3l/sec each.

the boiler/chiller will need to supply this load plus a small margin-- 10% is usually enough.

So the shunt loop is basically 9l/sec plus margin = 10l/sec.

IF the primary loop vol flow is lower than the secondary totals, the last secondary circuit will get reverse flow and a mixing condition occurs thus reducing the flow temperature for that circuit.

The pump duties are calculated independently of one another so long as the primary circuit is low loss i.e. less than 1m/sec or around 100Pa/m. (very loose conditions because I think it would work on higher velocities)

I think TA (Tour and Anderson)do a good book on this subject, but you need a week to read it (and even longer to understand it)

Friar Tuck of Sherwood

 

[lilliput1]

The total heat (MBH) in the main loop is distributed to the heating equipment directly on the main loop and to the secondary loops. If the secondary heating loop load is say 600 MBH and the design secondary loop temperature drop is 20°F then the secondary flow gpm = 600 x 1000 /(500 x 20 ) =60 gpm. If the main loop however has a design temperature drop of 30°F then the gpm that the main loop (3) way mixing control valve main HW gpm that will be directed to the secondary loop is = 600 x 1000 /(500 x 30) = 40 gpm.

So if the main loop temperature is 200°F supply, 170°F return while the secondary loop supply is 190°F supply and 170°F return, check the mix secondary loop downstream of the valve temperature T = ((40 x 200) + (20 x 170))/60 = 190°F secondary loop supply temperature which checks out.

 

[supaman79]

Thanks guys,

All of your inputs are extremely helpful, but maybe I should have asked my question in a little bit more detail. When I asked how big, I meant it's total length of the primary loop.

For example..say my primary loop is 41 gpm, serving (2) secondary circuits, one @ 26 gpm, and the second 10 gpm. To get my MBH out of my main loop, does this loop have to be say 30 ft, 50 ft, 60 ft? Catch what I'm saying? I guess the best way to ask it is how much volume of water does this primary loop need?

 

[friartuck]

I see what you mean. You think that the primary loop is a reservoir of water of which you 'extract' your secondary water. The answer is NO. The primary loop can be 2 feet long or 200 ft long. All you need to do is make sure the primary pump can handle the heating load (plus 10%). Liliput explains how to work out the flow rate.

Friar Tuck of Sherwood

 

[supaman79]

Thanks friartuck...you're always a help.