Primary/Secondary Piping Arrangement
Primary/Secondary Piping Arrangement
(OP)
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?
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?





RE: Primary/Secondary Piping Arrangement
RE: Primary/Secondary Piping Arrangement
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
RE: Primary/Secondary Piping Arrangement
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.
RE: Primary/Secondary Piping Arrangement
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?
RE: Primary/Secondary Piping Arrangement
Friar Tuck of Sherwood
RE: Primary/Secondary Piping Arrangement