hi-temp water reduction

[CountOlaf]

Hey all,

Problem: I'm using hot water in a standard air handler heating application where I've called for 200 deg F water in and perhaps a 20 deg drop through the coil for my heating needs. It turns out the existing hot water system I'm tying into varies in temperature from 200 to 300 deg's F. (say 215 at 170 psi and 316 @ 220 psi). If my coil or other associated components can't handle the elevated temperature (control valve, piping specialties, etc.), what are some simple options for reducing the temperature in the mech room before I get to the coil? I'm open to any ideas such as pressure reducing valves, heat exchangers, blending, etc.

Thanks in advance.

 

[IRstuff]

You could do what's done in automobile radiators, which is to provide thermostatic valve that runs the water through a heat exchanger if the temp is above a certain value.

TTFN

 

[CountOlaf]

Thanks IRstuff. I kind-of figured I'd need to do an h-x type of thing. I wonder if I can get one that's air-cooled with a fan, similar to a unit heater, that way I won't have to find city water or another fluid and find somehwere to dump the wasted water?

 

[KDMKE]

Rather than discard heat with a heat exchanger, I'd use a 3-way mixing valve to meter the high-temp water into a pumped loop that flows through the coil. The pump keeps coil flow constant to maintain heat transfer characteristics and to prevent coil feezing. (I'm assuming the coil potentially sees freezing cold air.) Check the required flow of the high temp water - the mixing valve may end up being quite small. Control the mixing valve based on coil leaving air temp. If there's no risk of coil freezing, you might get by with two 2-way valves in parallel - a small one sized for about 1/3 of the load, the other for 2/3. As coil load demand increases the small valve opens. When its 100% open and coil load demand continues to increase, the second valve begins to open. Since the control point is leaving air temp, but the coil load & incoming water temp vary, the control valve(s) will have to modulate to compensate.

Either arrangement saves heating energy as well as providing good temperature control.

 

[CountOlaf]

Thanks KDMKE. I just might be able to solve this with control arrangements as you suggest. I don't have the drawings and sequence of operation in front of me so I'll have to revisit them next week to be sure, but I currently am employing a three-way mixing valve at the coil and I do recall having the hot water run full through the coil during low mixed air/outside air conditions to prevent freezing. The coil has integral face and bypass dampers for modulation as well. With your suggestions above regarding the pump, are we still talking about routing the high temperature water through the coil, just at a reduced water flow rate?

 

[pennpoint]

CountOlaf

You are obviously taking a proactive posture in this situation. That’s good. But having the information regarding the variance in temperature and pressure, wouldn’t it be also proactive to verify that you might only have to change a few components to withstand the changes pressure and temperature and take advantage of the greater delta T.
The pressures and temperatures do not seem to be that extreme IMO. Step back and take another look at it. Keeping it simple may pay off in the long run.

New Years Greetings
pennpoint

 

[quark]

A ready made solution will be to use a hosedown station sort of thing. These are generally used with steam and cold water for washing applications but can work well with hot and cold water.

http://www.spiraxsarco.com/assets/uploads/PDFs/sb/P157_01.PDF

To optimize energy usage, you should mix hotwater with coldwater, reduce temperature and use in the AHUs. You may not be able to recover useful energy if you use any other heat transfer devices.

Regards,

 

[HVAC68]

The pressure (170 psi to 220 psi) seems to be a bit high in my opinion. Check out whether the existing coil can take that high a pressure.

If I have got my conversion right, cooling coills/heating coils are normally designed for working pressures of around 150 psi.

HVAC68

 

[lilliput1]

See Bell& Gosset Primary Secondary Pumping Application Manual. Setup using (3) way modulating mixing valve is shown on Figures 27 to 30. Setup using (2) way modulating valve is shown on Figures 31 to 33.
Design your system for 200-20=180°F return water tempeature from both the primary & secondary system. Thus the secondary system will be 200°F supply and 180°F return. The primary system supply will vary from 300°F to 215°F supply but design primary return for 180°F. Temperature rise at secondary therefore is 20°F but is 120°F to 35°F on the primary.
For say 500,000 Btu/hr load:
Secondary gpm = 500000/(500x20) = 50 gpm
Primary gpm with primary at 300°F = 500000/(500x120) = 8.333 gpm
Primary gpm with primary at 215¯F = 500000/(500x35) = 28.57

Check mix T at 300°F primary:
Mix T °F = (8.333x300+(50-8.333)x180)/50 = 200 Check!

Check mix T at 215°F primary:
Mix T°F = (28.57x215+(50-28.57)x180)/50 = 200 Check!

 

[CountOlaf]

This is my first use of the forum and I really do appreciate the varied and thoughtful responses. I work at a small firm where I don't have many colleagues to bounce problems/ideas off of. I hope I can help someone equally as well on a future thread. Because I'm such a newbie at this forum thing, I'm hesitant to hand out any stars sinced I don't want to offend any contributors (hope that doesn't offend anyone!).

lilliput: regarding your response, I will check out those B&G manuals when I return to the office on Monday. It's weird how we all maintain similar resources. Thanks for the detailed analysis. I assume your primary/secondary solution was similar in concept to what KDMKE was describing in the first part of his/her response.

Hopefully when Trane finally get's back to me on their coil capabilites/limitations, it will be a lot simpler matter. These ideas will help me have some "worse case" alternatives. Thanks again all.

 

[CountOlaf]

lilliput1:

I checked out that B&G section today. Now I see what you were getting at--you were right on. This does seem like the way to go if I find out that the coil can't handle these higher conditions. If I get to that point I may have to start a new thread to make sure I'm sizing the secondary pump correctly!? Thanks again for the help!