Heat Exchanger 1

[QUAVIET]

Sirs, I have a heat exchanger which is being used as a hot water heater for a tray washer. It has been undersized since installation. I have contacted the manufacturer and the only response I have received is for them to offer to send out a salesman. We do not want to replace this unit and have been making do. My question pertains to increasing the steam pressure on the steam side. It currently is regulated to 30PSI. and is controlled by a pneumatic on/off ball valve which is ultimately controlled by a RTD at the hot water discharge. This all functions properly. The steel shell of the steam side is rated at 150PSI @ 400F. The water side, which is a brass temp regulator is rated at 15PSI. It is adjusted to max hot water (no bypass). The specs which I have downloaded call for a steam pressure of between 2lb and 15lb. Since we are already operating the steam side at twice the pressure of the specs. Would there be any harm to remove the steam regulator and apply 100PSI which is the operating pressure of the boilers to provide more energy. I ask this because the steam side of the heat exchanger is rated at 150PSI. What has held me back is that I do not know if the coils are part of the steam side and included in the rating of 150PSI. The steam side is manufactured by Niles Steel Tank CO. Part no. 02-850. Also the following data: SH.THK .232 YR 1992, MIN HD THK .9, R.D FLAT, MIN HD THK .232, 2:1 ELP. Thank you for any advice.

 

[unclesyd]

It sounds like you may be limited on the tube side of the HE.

Comeback with a little more information before you raise the steam pressure.
What are the tubes made of and the area of the tubes?
What is the inlet and exit temperature on the water side?
How are you controlling the condensate?
Is there any corrosion/fouling in the tubes?
What is the configuration of the tubes, U bends, one pass, Etc?

 

[waskillywabbit]

IMO, "making do" and exceeding operational/design specifications are a recipe for an accident that gets someone hurt. Get a properly sized/designed heat exchanger for your application, which should have been done in the first place.

Brian

 

[pmover]

assuming the source of steam at the higher pressure & temp is not condition (no heat added/removed) when the pressure is reduced to a lower pressure for the exchanger, there is no benefit in increasing steam pressure to the exchanger as the enthalpy of steam at the higher p & t equals the enthalpy of steam at the lower p & t, other than increased steam mass flow. increasing mass flow may not be beneficial as the steam still needs to condense.

how is the exchanger undersized? by what process conditions is the undersized being based on?
-pmover

 

[QUAVIET]

The heat exchanger supplies the hot rinse water of a tray washer (pallet may be a more accurate word). It does not heat water for the wash which is a seperate heat exchanger. We must provide 180F rinse water for sanitary reasons. The only way I can supply this temp of water is to slow the flow of the rinse water through the heat exchanger by valving down the water flow. In fact I try and adjust this water flow to keep maxium water flowing through the heat exchanger in order to provide 185F and must keep an eye on seasonal changes on the city water temp. In this way the automatic water temp control of the chart recorder and the pneumatic steam valve are voided. In my opinion there is not enough water for a good rinse but as long as 180F water is printed on the chart recorder everyone is happy. Sometimes trays must be washed twice. I have experience with boilers and was not the one to install the 30PSI steam regulator when the manufacturer specs 2PSI to 15PSI. My questions are more for academic reasons. The heat exchanger tubes are copper. I make an educated guess that the heating surface of the tubes is 8 sq.ft. There is a functioning steam trap for condensate removal. If the condensate from the steam trap is allowed to drain to the floor instead of going to the condensate return line there is no improvement in performance. This condensate has a good flow. The steam trap in fact looks a little oversized so it is not the problem. The condensate return line functions properly. The tube bonding may be the limiting factor about raising steam pressure and the corresponding increase in temperature on the steam side. The water supplied to the water side is city water. The manufacturer specs indicate that the heat exchanger should just be able to do the job. The only other thing I can think of is there is some kind of insulation on the steam side of the tubes. I have not done a visual of this because of operational requirements. Have you ever seen this?

 

[pmover]

quaviet,

general comments...
the source of water should be clean and free of fouling material. since you state it is city water, that "should" be the case. however, it may be beneficial to get a sample and determine the contents.

heat exchangers are susceptible to fouling. in situation described, the fouling will likely take place on water side. fouling is dependent upon the cleanliness of the water supply. fouling could also take place on steam side, depending upon steam quality, which is dependent upon the boiler's water supply.

yes, to maintain the required water temp, it is necessary to establish a water flow rate to meet that temperature. in this situation, you've clearly indicate a reduction in water flow. but what is not known is the design water flow rate for the exchanger.

if you desire further assistance, it would be beneficial if exchanger design data can be provided. that is design inlet/outlet water pressures, temperatures, and flow rate AND steam/condensate conditions (actually you've provided them already, so can guess). by providing this data, individuals ought to be able to substantiate your claim or argument.

is this a new exchanger or existing?
good luck!
-pmover

 

[QUAVIET]

Thank you for your input pmover. The heat exchanger specs at 20GPM at 15PSI steam @ 180F with input water at 40F. I have made a visual of the water side and it was clean. This problem has been in there since installation in 1999. This has led me to believe that the problem is not a coating on the steam side of the tubes (unless a manufacturing defect or some tubes are blocked) but I have no other explanation. The flow of water I can produce at 180F is around 5GPM. This is a large descrepency. We have learned to live with it but it has always nagged me as to why we can't produce 20GPM @ 180F as spec even with 30PSI steam.

 

[unclesyd]

As stated you may have a dirty HE.

Another possibility is that you have collapsed some tubes due to overpressure or the tubes may have thinned due to corrosion.

Check your trap size and if too big install the proper capcity trap as too large a trap sometimes will not operate properly.

 

[GenB]

I worked in washer problems before and I suggest you supply hot water to the heat exchanger.
you can also add a hot water booster.
I just finished an electric hot water booster (stainless steel) 250 deg F. 100 psi pressure ASME Coded.
The system had a heat exchanger 180deg F and during cycles the water coolerd down, we added a 250 deg F booster to mix the exiting water, it worked perfectly.
You need to calc. GPM and time per wash, then if you need more advise. send to generalblr1@netzero.net or post it,
I will try to help.
ER

 

[rmw]

Your heat exchanger might be air bound. And, by air, I mean any non condensable gases, but I will generically refer to it as air.

All steam has some air in it. Not knowing anything about your steam source, I will still say that all steam has some air in it. The best deaerator made is not 100% efficient. Chemicals added for a variety of reasons can give off gases along the process. Air can diffuse into piping at steam leaks. I can go on and on. Air has to be considered when you are condensing the steam, and returning it to its liquid state.

If you have no means, or should I say, no good means of removing the air, then it just builds up in this "dead end" of the steam circuit.

Some traps are specifically designed to handle air, as well as condensate, others are only designed for only condensate, assuming the air is handled elsewhere.

And, for my part, I think traps are great condensate removers, but I don't like depending on them for air removal. Plus, another trap consideration to look at is if it is actually removing the condensate, and you don't have flooding, which would block heat transfer surface.

You need to either insure that your trap is designed for removing air, and is functioning, or obtain one that is.

Or, and I favor this more, you need to vent the heater at the cold end. (I hope your trap is connected at the cold end, as well.) You can vent it to a lower pressure steam header, or a condenser, or just to atmosphere. You don't have to vent much, and don't want to vent so much that you don't condense any of the through put, just enough to let any entrapped air escape. Just a 'whisper' of venting.

Regarding the other suggestions, pmover makes a good point that increasing the pressure would put more "pounds" of steam at a given enthalpy into your process, so that would be benificial. It would also push more condensate through the trap.

Unclesyd, thinning of the tubes, until they reach a point of leaking, helps heat transfer, as it reduces one of the thermal resistances in the overall U formula. It does reduce velocity, but he has already reduced velocity significantly to get his performance, so velocity does not seem to be an issue.

So, check for air binding, or flooding. Either would give your symptoms.

rmw

 

[QUAVIET]

Thanks again all for your valuable input. I have printed out this thread for management and for my files. I think we will look into this at our next shut down.

 

[rmw]

Duh!!! I just gave myself a dope slap.

It is 8 hrs after my previous post. Something pmover said in his 3/31 post about the constant enthalpy kept nagging at me, even as I went out to eat boiled crawfish.

You stated that your boiler header pressure is 100 psi, and did not state whether or not that was saturated or superheated.

Even if it is only saturated, when you regulate it down to 30 psig, it superheats 35 degrees F. That amount of superheat is enough to detrimentally affect the heat transfer in your Hx. Superheat will act line air, by blanketing tube surface while it is desuperheating by sensible heat transfer, so that it can get to its saturation to condense. Your case is worse if your boiler steam is superheated at all.

Even the best of Hx's, specifically designed for handling high superheat coming off of turbine extraction, namely utility steam turbine cycle feedwater heaters, which have specially designed sections or zones to reduce the superheat, so that the steam can pass to the condensing section at or very near saturation temperature, only have OHTC values of 100-160 for the DSH zone, and to get that, they have to pass the steam across the tubing with velocities so high that they have to watch out for damaging tube vibration. I doubt your Hx is designed that way.

Once you get to saturation, and condensing can occur, the OHTC's get into the 4-5-600 range, again, depending on your Hx configuration.

You have to get the superheat out, plain and simple. Your Hx will probably do just fine, even at a lower pressure, if you can bring saturated steam to it.

What I said about the air above, however, still applies.

Good luck!!!!

rmw

 

[QUAVIET]

To rmw, I do knot know what the properties are between superheat and saturated since my background is electronics. I am a licensed high pressure boiler operator but this was not discussed in the school I attended (20+ years ago). Are you saying that if the Hx is designed for 15PSI steam (low pressure boiler in Ohio) then it should be operated from the same? Our boilers are Scotch Marine set to 100PSI and there is no de-aerating or superheat except for the 35 you mentioned after the 30PSI regulator. If the Hx is indeed the wrong piece of equipment installed for our steam is there an easy and cheap cure? Eat some crawdads for me. Bob

 

[QUAVIET]

In re to my last post. There is about thirty feet of uninsulated inch and a quarter steam pipe which feeds this system. This may help account for the good flow of condensate I observe from the steam trap. This system is about 300 feet from the steam header in which the rest of the larger pipe is insulated. The last draw from the larger steam pipe which has steam traps is about 40 feet from the regulator.

 

[rmw]

Bob,

We are dealing with several different issues here.

First, let me say about uninsulated piping, that any moisture that forms in the pipe will be detrimental to your regulator, and will cut the tubes in your heat exchanger if it makes it all the way there.

Second, regarding the superheat. If you know someone who is a mechanical engineer, or a chemical engineer, (and there are a lot of other disciplines that had to suffer through thermo) see if they can explain the terms "adiabatic expansion," and/or "mollier diagram" to you. What it simply means that saturated steam, when it expands without transferring any heat across a PRV, or an orifice, retains its enthalpy, and this causes it to become superheated at the new lower pressure that it has been reduced to. I ran a steam tables check before I mentioned the amount of 35F. It could be part of your problem.

If your boiler system is such that it can be operated at 15 psig, instead of 100, (and I have had experience with scotch marine boilers with an either/or control setup) and, there are no other steam users in the plant that require 100 psig steam, then you will go a long way toward solving your problem by reducing your steam pressure to 15 psig. The Hx should be able to handle that without the pressure reducing regulator upstream of it.

Without the use of a deaerator, any dissolved gasses that are in your make up water get released into the steam system, and hence, the source of the air I was looking for above. There are corrosion issues that come into play here, as well.

You may have a situation of any of the above, or all of the above, problems exist, depending upon how hard the Hx is pulling on the system. Sometimes you may pull hard enough to overcome all the lack of insulation, and arrive at the Hx with superheated steam, and in others, when the steam usage is low, and the steam has had time to cool down, and maybe even condense a little in the supply piping, you may be cutting your regulator, and/or Hx to ribbons. That situation needs fixed. Insulation is cheap, and mechanical damage is expensive.

I think the heat exchanger, without any more particulars on it, is probably fine. It is your steam system that needs the attention, so that the Hx can function properly.

The crawdads were great.

rmw

 

[TBP]

I doubt either wet steam or superheat is the problem. I'd check line size after the PRV. A pretty good rule of thumb is the PRV is one size smaller than the correctly sized supply, and the downstream line is double the diameter of the upstream steam line. The 1-1/4" line upstream appears OK for the flow (GPM water X delta-T F X 500 = BTU/hr required). There's about 1,000 useable BTU in a lb of steam, so you require 1,400 #/hr to heat the design water flow. The 30 feet of 1-1/4" at 100 PSIG will handle this flow. Off the top of my head, I'd guess you require a 2-1/2" or 3" line downstream to handle the flow at reduced pressures.

Lose the on/off ball valve for temp control, and get a proper modulating control valve. Depending on how the steam lines are sloped or arranged, there should probably be trap ahead of the PRV to prevent a slug of condensate collecting while the washer is between loads. Get hold of a copy of "Hook-Ups" by Spirax Sarco. It's full of excellent info.

 

[QUAVIET]

The plot thickens. The unisulated pipe feeding the Hx is 1in. instead of the 1 1/4in. as I stated earlier. It is in the ceiling and I made a guess. Also when the insulation starts after 30ft. the pipe then travels 200 feet and somewhere in the insulation changes to 1 and one half inch. In fact this steam pipe makes a U turn around the whole plant after supplying process along whole way. It does neck up to much bigger pipe for these other processes about 300 ft from the Hx. The odd thing is that the 1in. pipe which feeds the Hx comes within inches from two 2 1/2 inch pipes dedicated to the wash and the air dry of the Hx? This 2 1/2 inch pipe is only a couple of feet from the regulator. The one inch pipe feeds a one inch 30PSI regulator and exits the regulator as one inch until reaching the Hx in one foot. The Hx is not insulated. As I read it it looks like we need to pipe into the 2 and one half inch steam pipe which is inches away. We then need to downsize the regulator to 3/4in. and change the pipe between the regulator and the Hx to 2in. My boss and I have already discussed a regulator to throttle the steam instead of a on/off pneumatic ball valve but I knew there was more to the issue. My only responsibility to the Hx is electrical but I like things to work correct.

 

[GenB]

With the amount of the heat ex heating surface, you will never operate the washer as intended,
you need accumulated heat to accomplish the task and you do not have it . you need additional quick heat that only power can do it,
electrical power as i mentioned before.
do your calcs on heat, heat loss using btuh, your washer calls for a tremendous amount of heat at one time, heat ex will give the heat overtime and not when you need it.
theoretical you can play with the #s but in practice, you can not see the problem.

I have a mahine which rolls pipe, it suppose to work with a 10hp motor.(mfrs specs and originally suited with)it does not, I installed a 20hp and it been working for the last 20 yrs.
I am sorry if I am confusing you.
ER

 

[QUAVIET]

To generalblr, I may be missing something from your post. This Hx only supplies rinse water. Earlier I mentioned 8 sq.ft. for the tubes. This was only a guess. The Hx as rated should be able to do the job. It cannot supply rated 20GPM @ 180F, only about 5GPM @ 180F. The Hx itself is physically the size of a bollard used around plants to keep trucks from hitting something.

 

[TBP]

A 3/4" PRV likely won't quite do the job. If the inlet steam pressure was 125 PSIG, it would, though. The length of pipe involved is a big factor, as well. The problems you're encountering sound more and more like they are related to pipe sizing issues, as opposed to the actual HX.

Find a local rep for Spirax Sarco, Armstrong, Spence, Hoffman, etc and get them to look at the piping arrangement.