[Bldg-sim] Steam Boilers to Reheat Convertor

[Mark Prince]

Hello all,

Here is my situation.

I have boilers generating steam.  The steam is used for process loads such as autoclaves and domestic hot water.  The steam is also used directly in my air handling units for humidification, and some of my air handlers have steam preheat coils.  In addition, the steam is being used at hot water heat exchangers for my reheat hot water system.

How should I model this in my waterside systems?

The way I have done this so far is that my primary heating loop is from two steam boilers.  Attached to this primary loop are the autoclaves/domestic hot water heaters as direct loads and the air handling unit humidifiers and preheat coils.  For my reheat loop I have created a secondary loop off the primary loop.  I modeled the reheat loop with 180 degree supply temperature with a loop design DT of 50 in accordance with ASHRAE 90.1-2004 G3.1.3.3.  I've also got the hot water supply temperature reset programmed in accordance with ASHRAE 90.1-2004 G3.1.3.4.  On my reheat loop I've programmed the "% Transfer to Primary" as 100%.

Have I modeled this water system correctly?

Here are my worries.

1.) The "% Transfer to Primary" indicates to me that I shouldn't be trying to create a reheat loop secondary loop off a steam loop primary loop.  Based on the DOE-2 help for the "% Transfer to Primary" this function operates on gpm's and loop DT's.  My steam loop doesn't have a gpm, but more importantly I don't have a delta-T on my steam loop.  Saturated steam temperature is 350 F, and the saturated condensate temperature (in theory) is 350 F.  So I don't really have a delta-T.  How do I accurately specify % Transfer to Primary under this type of system?

2.)  Also, if I specify my boilers as being "steam boilers" why does it want to know a "Design HW Temp" and a "Loop Design DT"?  You would think that by specifying steam boilers it would lock these two items out.  I realize that it may be trying to calculate pump energy off this, but I've already negated the pump energy off my steam loop by forcing the steam loop pump power at 0 kW.  There is no pump energy on a steam heating system.  Steam is self-motive.  Except for condensate return pumps, but this energy is negligible.

3.)  One last item.  Under the airside systems, for humidifiers and preheat coils, it allows you to attach these sources to "Steam Loop".  Now, under the waterside systems you can not call a circulation loop a "Steam Loop"  you can only call it a "Hot Water Loop".  So, when you attach humidifiers/preheat coils to "Steam Loop" is eQuest smart enough to attach that load to your steam boilers?

Any insight on this would be greatly appreciated.

I'm beginning to think that the most accurate way to model this would be to create two (2) hot water systems with two boilers in each system.  One hot water system would be steam boilers for the autoclaves, humidifiers, and domestic hot water.  The other hot water system would be hot water boilers for my reheat system.  The one thing I would have to do is just make sure the heat input ratio is the same for both boiler systems.

Thanks,

mark