Heat exchanger steam temperature

Pumpguy

Every once in a while I will see a 2 pipe vacuum return steam heating system that will have a hot water heating loop using a steam-to-hot water heat exchanger.

Can anyone tell me what a typical steam TEMPERATURE would be would be needed for this heat exchanger?

I would think this would depend on how the heat exchanger was sized, but if anyone has any experience or knowledge on this subject, i would appreciate their comments.

mikeg2015

Will totally depend on the HE size, water temp and flow rate.

Most mfg publish specs. Might be able to find them.

jumper

Are you asking about a storage type water heater with a steam coil or jacket?

Pumpguy

jumper said:

Are you asking about a storage type water heater with a steam coil or jacket?

Not usually.

When I run into these situations, and customer's complaint is poor system vacuum and vacuum pump performance, I frequently see high condensate temperature, high enough to negatively affect the performance of the vacuum pump, and overall system vacuum.

When I ask about operating steam pressure, which is directly related to steam temperature, I am told that the high operating steam pressure is needed to meet the needs of the heat exchangers for the hot water heating loop.

When I ask what steam temperature is needed for the HW heating heat exchangers, nobody can give me an answer, and the high condensate temperature issue goes unresolved.

I know with other types of heat emitters the EDR rules call for a 215 * F. steam temperature. Now, assuming the size of the HE is correct for the heat load, and the same is also true for the water flow rate, would this 215* F. steam temperature also apply?

EBEBRATT-Ed

@Pumpguy , generally heat exchangers are sized and sold for the job specifications whatever the designer decided. There not one size fits all like radiators or baseboard that have published ratings.

While some heat exchanger do have ratings in their catalogs they are usually sized for the specific job. Their capacity changes rapidly with an increase in steam pressure

Pumpguy

@EBERATT-ed, Thank you for your reply.

I understand what you are saying, but my question was about STEAM TEMPERATURE.

If you, or other members with actual experience, were sizing an HE for a similar application, a vacuum return 2 pipe system with steam heated hot water loop, what steam temperature would the HE be sized for?

jumper

Delta T for 105° hot water and 215° steam is 110°? So if you operate at 200° steam delta T is 10% less. I don't think that is significant. 200° steam is about 11 psia. Are you operating at higher vacuum than that?

Pumpguy

This is not a question about a specific job.

Typically, this scenario would occur when HEs and a hot water heating loop are added to an existing 2 pipe vacuum return system.

Since the EDR rules call for a 215* steam temperature at the system's heat emitters, and a typical operating steam pressure for a 2 pipe vacuum return steam heating system is 2 PSI, I would assume that the HE would also be sized for a 215* steam temperature.

So, based on the above numbers, is it logical for me to assume that the HE "SHOULD" be sized for a 215* steam temperature?

EBEBRATT-Ed

@Pumpguy , As far as temperature goes it rises and falls with the steam pressure (you already know this) your logic make perfect sense to me. Did it make sense to the original designer? I don't have a clue.

Buildings are seldom designed with a steam and a hot water heating system. The hot water zone was likely added.

Did the designer know that the steam should be at 2 psi? Not everyone does. If they did @Dan Holohan wouldn't be able to sell any books.

It's not like radiation. There are no "rules" for hx sizing that I know of.

Want to make hot water with low steam pressure put in a larger HX. Have high pressure steam available use a smaller hx.

I agree with you, to prevent overheated condensate and flash steam at the vac pump the hx should be sized for low pressure steam.

Pumpguy

@EBEBRATT-Ed , Thank you for your reply.

When I see these systems, nobody has a clue about the sizing numbers, only about the problem with the pump.

When I inform the customer the real problem is due to operating with too high a steam pressure which results in too hot condensate that is killing the vacuum pumps and their performance, they just shrug their shoulders and maybe turn off the vacuum pumps by putting the selector in the FLOAT ONLY position. It's very rare that they will actually try to operate with lower steam pressure.

I feel like a voice crying in the wilderness.

jumper

I don't understand the issues you guys are discussing. The steam powered hot water heaters I'm familiar with have controls to meter the steam according to hot water demand,both temperature & flow. So who cares what pressure the steam is at? Negative 2 psig or plus 2 psig, what difference does it make? And the condensate comes out only slightly above water temperature. Usually steam trap is unnecessary and not a pump either.
If the vacuum return pump creates back pressure against condensate return from hot water heater,I would deal with that problem differently than feeding HE condensate to vacuum return pump.Separate return or perhaps elevate HE.

EBEBRATT-Ed

@jumper with a heat exchanger the steam pressure has a dramatic affect on the hx capacity. Sometimes raising the steam pressure is necessary to heat the hot water load. Unfortunately when the load backs off you still have steam at a higher pressure than you would like causing condensate at a higher temperature than is good for the vac pump. Also the higher the steam pressure the more flash steam in the condensate lines and vacuum pump.

@Pumpguy , a couple of suggestions don't know if they are practicle for your job. #1 A condensate cooler (hx) to cool the condensate from the hx and use the heated water to preheat a dhw heater or tank.

or

A separate condensate pump without a vacuum pump and return the condensate from the hx to the boiler directly without going through the vacuum pump

Pumpguy

@EBEBRATT-Ed , Thanks again.

You have a complete understanding of the problem at hand.

And, yes, on more than one occasion I have recommended installing a separate condensate pump to handle the high temperature, and some customers have actually made this change.

As far as installing a condensate cooler, using the heat to preheat DHW, I have made this recommendation as well, but this seems to be too complicated and expensive to implement.

Thanks again to all that responded. Best wishes to all for a Merry Christmas, and all the best for 2018.

jumper

EBEBRATT-Ed said:

@jumper with a heat exchanger the steam pressure has a dramatic affect on the hx capacity. Sometimes raising the steam pressure is necessary to heat the hot water load. Unfortunately when the load backs off you still have steam at a higher pressure than you would like causing condensate at a higher temperature than is good for the vac pump. Also the higher the steam pressure the more flash steam in the condensate lines and vacuum pump.

@Pumpguy , a couple of suggestions don't know if they are practicle for your job. #1 A condensate cooler (hx) to cool the condensate from the hx and use the heated water to preheat a dhw heater or tank.

or

A separate condensate pump without a vacuum pump and return the condensate from the hx to the boiler directly without going through the vacuum pump

Sounds easier to have more generously sized HE with steam flow controlled for desired water temperature. Leslie & Aerco was selling such systems in the sixties.

EBEBRATT-Ed

@jumper, I agree but getting them to design that way and spend more $$$$ probably won't happen even if it would work better