Heat Exchanger
Heat Exchanger
(OP)
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.





RE: Heat Exchanger
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?
RE: Heat Exchanger
Brian
RE: Heat Exchanger
how is the exchanger undersized? by what process conditions is the undersized being based on?
-pmover
RE: Heat Exchanger
RE: Heat Exchanger
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
RE: Heat Exchanger
RE: Heat Exchanger
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.
RE: 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
RE: Heat Exchanger
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
RE: Heat Exchanger
RE: Heat Exchanger
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
RE: Heat Exchanger
RE: Heat Exchanger
RE: Heat Exchanger
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
RE: Heat Exchanger
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.
RE: Heat Exchanger
RE: Heat Exchanger
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
RE: Heat Exchanger
RE: Heat Exchanger
Find a local rep for Spirax Sarco, Armstrong, Spence, Hoffman, etc and get them to look at the piping arrangement.
RE: Heat Exchanger
RE: Heat Exchanger
as waskillywabbit stated, your situation is a disaster
waiting to happen.
Frankly, I am amazed, a 'licenced' boiler operator
you should be aware that operating plant incorrectly
is a prime cause of accidents and eould never operate
a system at twice the safe working pressure. Perhaps
your training didn't include disaster photo's.
Imagine the scenerio:
The hx fails, staff are injured, production stops for a long time.
Investigators discover plant was operated at twice rated pressure, the insurer denies liabilty due to operator error. I guess operator would lose licence, leaving yourself unemployed. long and expensive litigation
follows.
And how could one explain to the deceased's family.....
ummm, I was just trying to get the dishwasher
rinse cycle to work?
Too many good men have died already, if your employer/management are unable or unwilling
to bite the bullet, quit before someone is
seriously injured or killed under your watch.
Report problem to your local Health and Safety
they will shut down the equipment without
hesitation, again resulting in loss of
production for a long time.
my 2 cents....
cheers