Building hot water system form central plant (Delta T)

[xnetfly]

We are a large pharma company with a section of campus that is operating off of a central boiler hot water system. This is a secondary and primary loop situation. The boilers are producing 100 GPM of 175F water to the buildings. However, during the summer the building is not using (or very little hot water) and the total gpm out of the plant is close to 8 GPM. Small... Valves closed and so forth. Do not have all the temperatures handy but the issue is the return water due to load is so very low producing around a 50F delta. This is causing the primary loop to drop in temperature as well approaching a tripping condition of the boilers. My question is with a hot water system and no bypass or load during the summer what is expected from the delta T. It seems the heat loss through the piping from the supply side and the return side when the valves are closed in the building would be the same... IS this not the case.. Any ideas??

 

[trashcanman]

I don't see how a low return water is dropping the primary temperature. Isn't the primary AFTER the boiler(s)? Even if it came in at 75 Deg.F (assuming the boiler wouldn't have cardiac arrest), the leaving (primary) water temp would be 175 Deg.F, correct?
Do you know where the hot water is being used during the summer? A reheat system perhaps? If so, consider using electric reheat for 1 or 2 systems (can't be very many, with just 8 gpm) so you could shut down the primary system for summer. Good time for maintenance then. It doesn't sound very efficient to run a large primary system for just 8 gpm.
Another thing to consider is using a smaller boiler & pump for summer. Most boilers want a certain minimum flow through them to keeep them happy. Check with manufacturer.
How are you going from 100 to 8 gpm? With a throttling valve or variable speed pump?
The 100 gpm flowing water would have more heat loss than the 8 gpm flow, due to higher heat transfer coefficient. Prob not a whole lot less though.