Steam Heating of Domestic Hot Water

Guy_6

What if you used something like a Super Stor 80 or 119 gallon indirect, which can operate on live steam. The condensed steam (water) would have to be pumped back into the boiler, correct? What if you operated that pump off of the tank aquastat, actually using the condensate water to block flow in that circuit? You would still want a tempering valve on the DHW for safety purposes. I am not sure of this idea, as I don't know of anyone having done it, but the steam minded here may be able to either build this idea, or expose it's shortcomings. What do you think folks- will this work?

Jack Getkin

Steam Heating of Domestic Hot Water

I am designing a domestic water heating system using a brazed plate heat exchanger and storage tank to heat domestic hot water from 40 - 140 with 15 PSI steam. The equipment will be located such that the condensate will need to be lifted about6" - 8" to a point where it will drain by gravity. I see Dan's recommendation in chapter 10 of the "Lost Art of Steam Heating" (page 161) not to use a control valve to stop the flow of steam where condensate must be lifted to an overhead return because once the valve closes, there is not pressure to lift the condensate. I need to stop the flow of steam to the heat exchanger when the water in the tank reaches the desired temperature. So, how do I do this?

Thanks.

Brad White_9

Need a Lift?

Jack, How big a system is this? Using a BP HEX indicates to me that it is a small load. Is there a reason that the plate cannot be lifted to allow gravity to work?

If the system is large enough, why not use a shell and tube type? (Thinking less height overall perhaps; both will have traps, correct?)

If you cannot raise the exchanger, I see as the only practical option that you provide a condensate pump, either pressure (steam) powered or a small electric unit. I mean, for six to eight inches, (1/4 PSIG) certainly you can find some higher ground, can't you?

Jack Getkin

Steam Heating of Domestic Hot Water

Brad:

Thanks so much for your reply. The lift I refer to is actually at a step situated between the existing water heater location and the location of the condensate collection point. I suppose it is possible to relocate the BP heat exchanger, storage tank and associated controls to "higher ground" but the cost associated with extending the steam supply, hot water supply, cold water supply and return pipes to the new location could be problematic and costly. Then there is also the issue of regulatory space around the heater for service and maintenance. I like the condensate pump idea and will explore that.

There are two very large existing heaters that presently lift the condensate the distance I indicated and they are equipped with control valves. Dan's recommendation makes sense but I wonder why the existing installations have worked for 60 years? One of the heaters failed some time ago and the other is on its last leg. The facility also dramatically reduced its domestic hot water load so I will be proposing a significantly smaller unit.

I guess another possibility would be to move only the BP heat exchanger to the "higher ground" and leave the storage tank near the existing heaters. I will also explore that option.

Thanks for your thoughts.

jack Getkin

Brad White_9

Control Valves and Lift

Jack, Hi

The only reason I think that the heaters worked for 60 years is that the condensate never actually left the heaters at the end of a cycle. The next cycle forced out the old condensate and replaced it with a new slug. If the control valves had a slow opening characteristic this may not have been a problem. If fast acting, water hammer would be the result. Just conjecture on my part; I have not seen it so cannot really know. Also, older equipment must have been made of sterner stuff to withstand the occasional condensate torpedo.

As far as raising the BP, I would do that, but I agree with your last sentence in that you should not need to raise the tank also. The water side being under pressure and not gravity-dependent of course.

The BP exhanger- can you also get a horizontal version of that? Wide and squat? And do not forget the strainer upstream, really fine mesh, even if the CV has one upstream of it as it should anyway. BP exchangers can make excellent strainers themselves.

Tony Conner_2

Watch...

... the steam pressure at the HX inlet. You won't likely have 15 PSIG at the inlet to the control valve, let alone the HX. (If the steam safety valves lift at anything over 15, you're very probably into an ASME Code installation.) Best case will likely be 12 PSIG at the inlet to the control valve, then whatever the pressure drop is across it. You may only have 7 or 8 PSI at the inlet the HX, and maybe less. With low pressure steam, there are often problems getting enough #/hr stuffed through the HX nozzle.

Plate HXs with steam - not my favourite combo.

Tom R.

A float & thermostatic trap

should work with 15 psi steam and a thermostatic control valve. By nature, the valve does not open quickly enough to cause water hammer, and while in operation, the trap will discharge condensate at a constant quantity equal to the steam entering. With the lift you mention, you only need to overcome .25 psi in the return line.

Christian Egli_2

A pill for claustrophobic steam

Like you Jack, I see such valved lifts in operation without apparent problems but I agree, there can be no lifting without open steam. I don't know what a final ruling might be on this.

I do know the addition of a vacuum breaker chases away a lot of problems and often prevents the violent destruction of the unit and its trap. I think it also helps to add a good check valve on the condensate line so that you don't backfill your heat exchanger, a check on your lift should be there already. It is also important to select the right sized trap.

Of course the electric condensate transfer pump always works, it's foolproof, but somehow, for a lift of only a few inches I find it heavy handed. Here are six more ideas (on top of simply not worrying too much about what the valve might or might not do) which I hope you'll enjoy for their exoticism.

First, instead of an electric pump, a mechanical pump (either steam or compressed air powered) could do without the impeller pump. It is the alternating return trap of the old and still widely available as pumpless pumps. But, of course it isn't really cheaper to buy.

Second, you mention there won't be much domestic water use anymore. Perhaps simply pouring the condensate down a drain might not be such an unreasonable waste.

Third, you say you have a dry return over head that is limiting you, but you don't say where your boiler water line is. If your altitudes work out, you could very simply take your condensate outlet and branch it into two pipes right where it leaves the heater. One upward branch will become an air-only dry return which you connect to your existing overhead return. The second downward branch will become a brand new wet return you'll plumb any simple way you wish back to your boiler or nearest low level dry return. Since it is a wet return, slopes need not be a concern, only worry about a stacking head (of maybe one inch) to get condensate flowing.

Fourth, this is a nice one too, since you're going to have an electric remote controlled valve on your steam inlet that will isolate your coil and perhaps trap some condensate in it, why not install a second electric remote controlled valve as a take off on the condensate outlet, teed in before the trap, and opening to the atmosphere.

When your steam valve closes, this discharge valve opens and releases whatever problem causing condensate is trapped in there. It releases it to a drain or even just a bucket since we really don't expect much stuff to come out here. In this set-up a vacuum breaker could be superfluous while the backflow check valve is still critical.

Fifth, there are such hot water faucets that make you instantaneous hot water. They look like the cappuccino nozzles at Starbuck, they are fed one pipe with live steam and another with dead cold water. You turn it on, and it raises your hot water from beyond. This may be a perfect solution to your hot water downsizing situation.

I think that's enough for now. Perhaps something here will suit you. Best of luck.

Christian Egli_2

There is a no. 6

I am thinking as quick as I can, I miscounted my paragraphs in the earlier post. So, here is more:

Sixth, electroplating and metal finishing processes use plate or mattress like heat exchangers that are fully immersed into a bath. The inlet and outlet are both situated above the coil, and the so is the trap. The lift is the depth of the bath. There are a few sizing tricks to make it work just fine.

Whew, good thing I didn't come up with a 42 count solution.

Guy, your indirect tank use is just fine, but you still need a valve to stop the steam from arriving and condensing. Preventing the condensate from leaving does not prevent the steam from condensing (and all your heat comes from this condensing). Further more, stopping a pump does not stop the migration of fluid through it, particularly not condensate that is being bullied through by wild steam.

Steam would have no trouble traveling through a city full of dead-ends and culs-de-sac. Hot water, in contrast, needs nothing but one-way streets to be happy. See how the application of red lights in both cities gets to stop the traffic?

Back to the valved lift.

There is a hotel I go to. It has renovated convectors that get their heat from steam. These convectors are very low to the floor and the condensate has to lift itself about 3 inches to get out the old pipes. No vacuum breaker, no check valve. It bangs just a very little on the opening of the steam valve. You see the problem very plainly when you take the thing all apart, that is also when you hear it the most.

And there is just never nothing to watch on hotel TV.