<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
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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@tess-inc.com
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