sewage heat pump issue

[Lc_shi]

Hi sirs
Sewage heat pump can save much energy and there're some project here in my country. I know there's no very special tech but I don't know what type heat exchanger is for the sewage water.
Who've done this kind of job would share us something,pls


regards
LC

[NoNickName]

Please explain what type of sewage it is, and how much solid is inside of it.

[US Iceman]

Hi LC,

The project I worked on used a simple shell & tube heat exchanger for the chiller and condenser (about 15 years ago).

The water that was flowing through the chiller was the grey water (I think this is the correct term) from the sewage treatment system. It is essentially clean water (all solids removed), but non-potable.

All of the solids and and other material were removed before the water flowed to the chiller.

[Abby Normal]

Sounds like you were reclaiming heat from a clarified or treated effluent.

I remember a documentary with some footage from Sweden where they were talking about reclaiming heat from sewers.

Some documentary got into bio gas from manure as well.

[US Iceman]

Yep, the sewage was the clarified effluent discharge.

Sweden does have a very large heat pump system which uses centrifugal compressors in the heat pump. Basically, it is just a big centrifugal water chiller.

On the bio-gas system... Was this being used for co-generation and combustion turbines? There might be some refrigeration required to condition (dehumidify) the methane before it is used. Have worked on some of these too.

[Abby Normal]

Yep, the sewage was the clarified effluent discharge.

Sweden does have a very large heat pump system which uses centrifugal compressors in the heat pump. Basically, it is just a big centrifugal water chiller.

On the bio-gas system... Was this being used for co-generation and combustion turbines? There might be some refrigeration required to condition (dehumidify) the methane before it is used. Have worked on some of these too.

The Biogas was from manure on a large dairy farm maybe 3,000 head.

It was treating the waste and producing the gas to replace propane. They seemed to find ways to use up as much of the biogas as they could but ended up burning a lot of it off.

They added greenhouses and heated with biogas, they ran stills made enthanol, but I do not rexall them generating electricity with it.

A starch plant in my hometown was forced to treat their effluent. They used an anerobic digestor to treat the waste and appease environmental concerns. It produced a substantial amount of gas, but it did not pay for itself. Was a smaller scale than 3000 crapping cows I guess.

[US Iceman]

If I recall this properly, methane is a bad greenhouse gas. Supposedly worse than CO2.

I wonder why cows were not legislated in the refrigerant protocols?

[NoNickName]

If I recall this properly, methane is a bad greenhouse gas. Supposedly worse than CO2.

I wonder why cows were not legislated in the refrigerant protocols?

GWP of CO2 is 1 by definition of GWP
GWP of CH4 (methane) is 23
GWP of R134a is 1300

GWP =global warming potential

[Lc_shi]

This system uses non-treated sewage water,only with two levels filter before entering heat exchanger. I'm not sure if the normal shell-tube HX is no problem.

regards
LC

[US Iceman]

Hi LC,

I would say the shell & tube exchanger is more forgiving in this application as the tubes would be easily cleaned. The project in Sweden does have shell & tube heat exchangers used on it I believe.

If there is a chance of minor particles or sediment, the tube velocity should be maintained at least 6 to 8 feet per second (1.8 to 2.4 meters per second) to provide a cleaning action on the tube ID.

The fouling factor should also be increased above the limits used for HVAC applications to also provide some additional run-time between cleanings too.

If the shell & tube exchanger is fabricated with channel heads and covers, the cover plate can be removed quickly to allow the tubes to be cleaned quickly.

The application of any heat exchanger is based on the application. If the fluid has solids or will accumulate material, then the system is dependent on the filters used. There are some centrifugal separators that can be blown down on a regular basis to maintain effectiveness of the seperators.

If filters are used, perhaps they should be provided with mechanical scrapers to keep the filters clean for longer periods of time.

Do you have any written material in English that may describe the heat pump you are thinking of?

[US Iceman]

Here are some links for more information:

http://www.stowa-selectedtechnologie...eat.Pumps.html

http://www.gryaab.se/admin/actions/u...2-10%20EGü.pdf

[Andy P]

The fouling factor should also be increased above the limits used for HVAC applications to also provide some additional run-time between cleanings too.

It used to crack me up that the US tables of fouling factors started with distilled water (FF=0.0005 if I remember correctly) and went all the way up to "The Chicago Sanitary Canal" at 0.008 (imperial units). The second worst after the CSC according to the table was the East River, NYC - must be all the ex-mobsters swimming with the fishes in their concrete overshoes!

Another laugh is that the Americans call imperial units "English units" - we've been blaming the English for all sorts of things for years!

Thanks for the memories
Cheers
Andy P

[ernestlin]

Hi, Mr Sze. I wish those links will be helpful. http://www.geoexchangebc.ca/pdf/Oslo...heat%20pump%22

http://www.georgiastrait.org/CAW/Res...heat%20pump%22

[Abby Normal]

Supposedly the worst greenhouse gas is water vapour, when you consider how much there is

[US Iceman]

It used to crack me up that the US tables of fouling factors started with distilled water (FF=0.0005 if I remember correctly) and went all the way up to "The Chicago Sanitary Canal" at 0.008 (imperial units).

I've seen similar tables as to the one you describe. If I remember right, they date back to about the 1950's or 60's. And, they were very, very conservative.

I guess I'm a little curious about the comment though. Do you think I was in error by suggesting a slightly higher fouling factor to allow for increased run-time between cleanings?

It is common practice to make allowances for differentiating between the fouling factors used for open and closed loop systems and potential fluid contamination on process systems.

I suppose I would like to hear more about your concerns Andy.

[NoNickName]

I guess I'm a little curious about the comment though. Do you think I was in error by suggesting a slightly higher fouling factor to allow for increased run-time between cleanings?

No, you were not in error. A customer once ordered a seawater/water chiller, so we made it with a special condenser.
They called us soon after it was commissioned, because it was not working. At first glance we put the blame on the condenser.
We removed and opened it.
We found toilet paper and algaes inside.

At the end of the day we discovered that, yes it was sea water, but pumped from a busy harbour without any gross filtration.

[US Iceman]

...but pumped from a busy harbour without any gross filtration.

I believe that was the point in the old fouling factor tables mentioned previously. I think the message here is, water is not always clean. Even closed circuit piping systems.

I also believe the importance of the fouling recommendations is not the absolute value specified, but that an investigation should take place to determine the exact condition of the water.

At the very least, some questions should be asked.

Sometimes we learn this the hard way. Some separate processes may require higher fouling factors to increase the run-time between cleaning.

This issue of fouling factors is very interesting. They do add expensive heat transfer surface and make the exchanger larger. In a competetive situation, it works against you trying to sell the equipment. Especially if the other companies do not do this.

That is why I think it is important to explain why you are adding it (fouling factor) and what benefit it adds.

Thanks for the feedback NoNickName. I would like to hear from others on this too.

[Andy P]

I guess I'm a little curious about the comment though. Do you think I was in error by suggesting a slightly higher fouling factor to allow for increased run-time between cleanings?

Hi Iceman - thanks for the comment. This is a tricky subject and I'm no more expert than average. In general I totally agree that river water, harbour water and "waste water" should be assumed to be dirty and the design should include an allowance for fouling. However all that the allowance achieves is a lower figure for overall heat transfer coefficient than you would otherwise have, and hence as you said, a bigger surface area. At first this is clean so the heat transfer is actually better than design and the evap temperature will be a bit higher - then as the cr*p builds up the TD on the heat exchanger widens because the performance diminishes. You are spot on: this does not eliminate the need for cleaning just makes the time interval between cleans a bit longer. However in a bad situation that might mean once every eight days insted of once every seven day!

My concerns about this approach are:
1) as the surface fouls and roughens I suspect that the rate of build up increases so paying large amounts for a lot of extra surface might only buy you a little extra time.
2) keeping the tube velocity up is also good practice, but the combination of higher velocity and particulates in the water could lead to rapid erosion. If you're going to do this then use a tougher tube material such as cupronickel or admiralty brass rather than copper (for *****s) or stainless steel rather than carbon steel (for ammonia)
3) one problem of increasing fouling factor and getting a bigger heat exchanger is that the flow area probably went up for the revised selection, and unless the number of passes increased the tube velocity would therefore go down.
4) for me the best approach is good filtration upstream, with tough tubes and fairly high velocity. Unfortunately the filters probably cost more than the increased surface. Ideally a pair of filters with an alternate automatic backflush arrangement to avoid the need for manual cleaning. With this arrangement I'd go for a moderate fouling factor of 0.002 hr.ft2.F/BTU (0.00036 m2K/W) or so

Cheers
Andy P

[Lc_shi]

GWP of CO2 is 1 by definition of GWP
GWP of CH4 (methane) is 23
GWP of R134a is 1300

GWP =global warming potential

Maybe ODP is more concerned
CH4 ODP = 0
R134a =0
R22 = 0.034
ODP = ozone depleting potential

[NoNickName]

And: who's going to clean the cr*p out of the fouled damned shell and tube?

[US Iceman]

My concerns about this approach are:
1) as the surface fouls and roughens I suspect that the rate of build up increases so paying large amounts for a lot of extra surface might only buy you a little extra time.
2) keeping the tube velocity up is also good practice, but the combination of higher velocity and particulates in the water could lead to rapid erosion. If you're going to do this then use a tougher tube material such as cupronickel or admiralty brass rather than copper (for *****s) or stainless steel rather than carbon steel (for ammonia)
3) one problem of increasing fouling factor and getting a bigger heat exchanger is that the flow area probably went up for the revised selection, and unless the number of passes increased the tube velocity would therefore go down.
4) for me the best approach is good filtration upstream, with tough tubes and fairly high velocity. Unfortunately the filters probably cost more than the increased surface. Ideally a pair of filters with an alternate automatic backflush arrangement to avoid the need for manual cleaning. With this arrangement I'd go for a moderate fouling factor of 0.002 hr.ft2.F/BTU (0.00036 m2K/W) or so

I agree with all of this and the methodology completely.

I think you will agree this is a tricky situation. The harder tubes (and/or possibly greater wall thickness) increases the wall resistance + the increased fouling factor will increase the exchanger cost.

Personally, I think I would investigate a prime surface ID for ease of cleaning and perhaps an exterior tube surface enhancement to decrease the exchanger size. However, this would all be based on the exact nature of the resulting film coefficients and the penalty forced by the increase in resistance of the other "corrections" (wall thickness & material conductivity).

This would make an interesting review of various tubes, materials, and surface enhancements, in an attempt to have the film coefficient ratios (inside to outside) be approx. equal to the surface ratio (inside to outside).

Even with the design methods used, the continued good operation of this would be predicated upon a regular periodic inspection and possible cleaning of the tubes.

This type of application, while a chiller, is a long way from the usual field of HVAC systems.

[US Iceman]

And: who's going to clean the cr*p out of the fouled damned shell and tube?

Well... That is part of the issue. Knowing the tubes have a high potential for excess fouling, as a designer you can only do so much.

Brushes that can be cycled through the tubes can be purchased. Filters can be installed.

However, the owner has to ensure the equipment is maintained and operating correctly as the designer intended.

I would rather over-design a system and tell the owner why, than explain why it doesn't work after it has failed.

[Andy P]

I think you will agree this is a tricky situation. The harder tubes (and/or possibly greater wall thickness) increases the wall resistance + the increased fouling factor will increase the exchanger cost.

Wall thickness: that's true for st steel vs carbon steel, but not such a big deal for cupronickel as far as I remember.

Another thing to be wary of when using higher fouling factors is the particularly bad effect of fouling on enhanced tubes. The enhancement gives greatly improved performance when the tube is clean - but the higher up you are, the further you have to fall. If using internally enhanced tubes (eg rifled or wavy) I would put a much heavier fouling factor on (maybe three times as much) - which of course cancels out the benefit of enhancement in heat exchanger size. Better to stick to plain tubes if fouling is a concern

Cheers
Andy P

[Andy P]

And another thing....(I really ought to go to bed!) We had a seawater condenser fail on a quayside job in Norway. When it was opened up we found severe erosion plus a relatively fine grit, which turned out to be the remains of seashells which had gone through the seawater pumps and been smashed up. A fine filter wouldn't have caught them after the pump, but a rough basket upstream would have prevented the problem. They were incredibly abrasive, and really ripped the tubes. You can imagine the rest of the story....

Cheers
Andy P

[US Iceman]

The enhancement gives greatly improved performance when the tube is clean - but the higher up you are, the further you have to fall. If using internally enhanced tubes (eg rifled or wavy) I would put a much heavier fouling factor on (maybe three times as much) - which of course cancels out the benefit of enhancement in heat exchanger size.

You said it better than I tried to. It's been too long since I have really had to do this sort of work.

Better to stick to plain tubes if fouling is a concern.

That's probably about the best advice a person can get for this type of service.

Have you ever used the HiTran inserts from CalGavin? These little inserts look like paperclips in a string. I have heard they tend to provide a self-cleaning action in the tube ID. I would be interested to learn the experiences of others on this item.

[Lc_shi]

i believe good ideas is the premise of the new product.
Peter can apply for a patent

regards
LC