Am I calculating the heat gain correctly?

[DesignerMike]

I have a customer with a DI water system. The cold loop (ambient temp) keeps heating up during times of low usage (nights and weekends). I predicted this, but they thought it would be insignificant.

I need to add a cooler to the loop at some point and am working on sizing it. I have only three sources of heat as far as I can see; 2 hp centrifugal pump on ozonation loop + 10 hp centrif pump on main loop, and 3400 watts (4.4 hp) from a UV unit. I figure that WORSE case scenerio I am introducing 16.4 hp of energy to the water system. This would be if ALL the hp from the pumps is being converted into heat from line and impellar friction. I calculate that I should be oversized with a 3 ton refrigeration unit and an exchanger.
I have a salesman that seems to think that they need a 25 ton unit??? I am doubting my numbers a little bit?

Thanks

 

[Montemayor]

Mike:

I think the salesman wants to sell you what he wants to sell, not what you need. I would normally design for 75% of your total electrical load as top, maximum heat pick-up; but I don't know what characteristics of operation you have. Like you, I think your 16 hp design load is the worst case - unless you're picking up heat from something else.

But you have the facts on your side. I recommend you measure the observed heat pickup from the data you can get:
the top water temperature, the water circulation rate and the initial water temperature. Even if you have to guess at the water rate, you're going to come up with a better and more accurate figure than your salesman:

Heat pickup = W * Cp * (T2 -T1), Btu/hr

where,
W = Water rate, lb/hr
Cp = 1.0 Btu/lb -oF
T2 = final water temperature, oF
T1 = initial water temperature, oF

Hope this helps support your position.

Art Montemayor
Spring, TX

 

[DesignerMike]

Thanks for the confirmation Art.

I didn't think I was missing anything....but wanted some confirmation.

I did another quick calculation, and determined that if I was introducing that much heat into the system, in 10 hours my water would be something like 475 degrees!

I'm pretty sure my conductivity probes don't draw 100 amps each

 

[IRstuff]

What's the change in temperature within the facility during off-hours? I assume that the flow is continuous, except at the drops? Also, is the DI running near anything that's higher temp during the off-hours?

As with Art, you need to get a handle on how much heat is actually being introduced into your system. Might be a good idea to see if you can determine where the heat is being introduced into the system.

TTFN

 

[GenB]

This is from yourposting...
centrifugal pump on ozonation loop + 10 hp centrif pump on main loop, and 3400 watts (4.4 hp) from a UV unit

You may want to start by converting hp and kw to btu/hr,
by the way 3400watts converts to 1/3 hp or merely approx. 11,000btu/hr.
Then add all the btuh heat and supply
the same or more of cooling; again in in btu/hr
ER

 

[IRstuff]

What conversion factor are you using for hp?

I get 745.7 W/hp, so 3400 W = 4.56 hp.

TTFN

 

[DesignerMike]

I'm not too concerned about an actual number at this point. I wanted to make sure I wasn't missing something.

The 25 ton and my 3 ton were so far apart that I was concerned.

I get approx 40 degree increase in water temp over the weekend in 1400 gallon of water, which makes sense.

I'll oversize a bit and go for it.

Thanks for the responses.

 

[GenB]

IRstuff
DesignerMike stared with compressor hp, then switched to heat/cool watts to hp,(heat form UV)
heat/cool converts to from watts to btu/hr, and I think that was his mistake in calculation, I also think he figure it out/find out about the mistake and solved his problem.
I may be wrong.
ER

 

[IRstuff]

Actually, I was questioning "3400watts converts to 1/3 hp"

TTFN

 

[GenB]

you too...

 

[25362]

A 40F increase for a mass of 11,600 lbs of water over a period of say, 35 hrs (a weekend) would mean 40x1x11600/35~13,300 Btu/h i.e., barely more than 1 ton refrigeration (=12,000 Btu/h).

 

[force27]

Does your loop have a heat exchanger to self sanitize your loop during off hours? If so, perhaps you are getting steam leak-by on your loop heater that is only showing up when your water turnover is low. This could be a contributor.