[TRNSYS-users] Water Heater - Water Draw Forcing Function (variable)

[Jeff Thornton]

<I created a model for a water heating system and I currently use a
Type14b forcing function to set my water draws. This forcing function
takes the variable water mains temperature from the weather input and
forces the mains water (tap water) into the water heater at a set
schedule and rate. The problem I have with this is that now the water
exiting the water heater is fixed at the 120 degrees F from the water
storage tank, and I want to use a tempering valve and set the flow rate
of the water to the mixture of hot and cold water instead of just the
hot water. I'd like to use a Type11b tempering valve to mix the 120F hot
water from the tank with water from the mains (which varies in
temperature) and output water at a fixed 105F. This means that the flow
rate exiting the hot water tank is less than the flow rate after the
tempering valve, since some cold water is added. (water from the mains
comes in both to the storage tank and again to mix with the hot water
exiting the storage tank) Is there a way where I can use a forcing
function to set the mixed hot/cold water coming OUT of the tempering
valve instead of using it to push water into the storage tank from the
mains. The water temperature of the hot water (in the tank) will be
fixed at 120F, but the cold water from the mains that mixes with the hot
water from the tank will have a variable temperature. So, while the
overall flow rate after the tempering valve will be constant, the flow
rate from each the hot water tank and the mains will be variable since
the mains temperatures change throughout the year. The Type14b forcing
function only allows me to 'push' water into the storage tank, not
'pull' water out of the tempering valve after it has been mixed with
both hot and cold water. Can you please suggest what forcing function to
use and in what order?> 

Tom, 

You can accomplish your goals with a
forcing function for the mixed hot water draw, a diverting valve, a
mixing valve and a rather simple equation. If you write the energy
balance at the tempering valve you get: 

Mdot_hot*Cp*Ttank +
Mdot_cold*Cp*Tmains = Mdot_draw*Cp*Tset 

If you define gamma as the
fraction of the draw that flows through the tank, then 1-gamma is the
fraction that bypasses the tank and gets mixed back in at the tempering
valve. Then: 

Mdot_hot=Mdot_draw*gamma 

Mdot_cold=Mdot_draw*(1-gamma)


Subbing these two equations into the energy balance you'll end up with
one equation and one unknown (gamma). The calculated value of gamma is
then sent to the diverting valve and the diverting valve outlets go to
the tank and the tempering valve (mixer). You have to watch out for
divide by zero and >1 and <0 situations but it's easily accounted for.
We actually have a model that does exactly what you need (Type 953) that
we use all the time in cases exactly like this. The model simply
replaces the energy balance equation for gamma. It's part of our
controllers library if you're interested. I've attached a simple
schematic showing how it all works. 

Jeff 

---

Jeff Thornton


_President - TESS LLC_ 

_22 N. Carroll Street, Madison WI USA 53703_


_Office: (608) 274-2577 Fax: (608) 278-1475_ 

_www.tess-inc.com_


_E-Mail: thornton at tess-inc.com_ 

 
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