11 replies to this topic

hai, to condense the low pressure vapor of about 120 tons/Hr @ 65 DEG C ,presently a finfan condenser is used.
It consumes a very huge electricty consumtpion of about 3 MW. is there any alternative way to do this. we have to do this as we need to recover the water.
Like heat recovery option, utilise this vapor in absorption chiller etc.

[Art Montemayor]

Habn:

I know all too well that it is customary to describe the product of water vaporization as "Vapor" in the Romance languages. It probably is the same in Arabic. However, in the Anglophone world it is still understood that the product of vaporizing water is STEAM – not "vapor". By using the term "vapor" you have created a dilemma for a majority of our readers. Do you have a pure water-steam system? Or are you dealing with a mixture of other compounds that have their respective vapors? Please be specific and identify the fluid(s) that you are dealing with. This is a very important point because you talk of recovering the "water", but you previously said that the "vapor" exists at 65 ^oC. That means you must have a vacuum – if you are alluding to a water system. Do you understand my point?

3 MW is a lot of power to be consuming and I can appreciate your concern for trying to reduce this to a minimum. However, please give the subject the same importance that we on the Forum give it: furnish ALL the basic data and a detailed explanation of the process involved and the scope of your work. That way, we can be the most help to you.

Await your reply.

The process is in a sugar refinery. the sugar syrup to be concentrated to become crystal in which moisture contents are removed by the evaporator & vacuum pan. In this process the sugar syrups are heated at low temperature of 80 DEG C under vacuum . during this heating process the moisture will be removed and hence vapors at low pressure of 250mbar(abs) @ 65 DEG C is send to the finfan condenser to condense and recover the water.

i hope i had clarified about the process.
guide me the best way to reduce the energy consumption

[riven]

Ok

You are feeding to an evaporator that is working under vacuum of 250 mbara and 65 C. From the steam tables the water will be a saturated vapour.

I assume these vapours are being passed through a condenser services by a low temperature fluid (chilled water maybe). This will produce your vacuum and remove your vapours.

Thus the only way to reduce your energy is to look at the service fluid system and see if that is using alot of energy. If the COP on your refrigerant cycle is below 3, you might consider replace and upgrade the chiller.
You may also consider the need for a chiller considering the relatively high temperatures and pressures involved.

no the saturated vapors are passed thru a finfan aircooled condenser where its cooled down to recover the water.
tell me the alternative way to minimise the energy

[riven]

no the saturated vapors are passed thru a finfan aircooled condenser where its cooled down to recover the water.
tell me the alternative way to minimise the energy

Ah you ee the usefullness of infornation (aircooled).

Ok do the maths. 120,000 kg/hr steam at conditions has a hfg ~ 2350. Thus the capacity of the cooling equipment is (120,000*2350)*(conversion factor kJ/hr to MW) is 78 MW.
You use 3 MW to do this so COP is 78/3 which is 26.
Using a COP of 50 (cooling towers are typically around this) would reduce your energy use to ~1.5 MW. So you could look at improving your COP if possible.

As for other processes for doing this, I have only one idea. A single stage steam ejector could be used to remove the vapours and generate the vacuum. However this is only useful if you can recover the heat from the steam you will need to supply to the ejector. And even then, it is probably unlikely.

thanks Mr.Riven,

i am thinking of to have a thermo/mechanical recompressor, i can raise the pressure and utilise this steam in a process or absorption chiller.
is it economically viable.if yes which is better thermo or mechanical compressor

[riven]

Ok now we are getting somewhere.

You are producing a lot of steam. You want to recover this heat but it is low pressure and thus low heat value.

You can as I said using a steam ejector. This is essentially the same as using a thermocompressor. I think however that this is unlikely but could be worth doing some sums. However I would consult Croll Reynolds as the equipment you would need would be very large and probably custom build. What you would be testing here is whether it is more economical to supply the ejector with stream to recover your low pressure stream or just throw away your low pressure steam. I think the latter would hold.

(It may be economical to use the steam as your low grade steam supply if you have many low temperature heating requirements?)

The problem with mechanical compressors is twofold. Firstly there are capacity issues and getting the required pressure rise would be problematic. You would be using a vacuum compressor.
Secondly you are recovering low grade steam using electricity (a lot of electricity) . Common sense would dictate that this is not clever.

So my initial assessment stands. Check to see if the COP of your current air cooled exchanger can be improved. If it cannot it is unlikely that much more can be done.

[pawan]

I would first like to know the following.

1. Heating source (Fluid)
2. Heating source Temp.
3. How do you create vacuum in the system - Water ejectors etc.
4. Are they connected after finfan cooler?.
5. What kind of other heating / cooling requirement exist in your plant.
6. Cost of steam / Te???
7. Cost of Power/ Unit???

These figures can give me an idea for a better complete solution.

To Riven

The problem with mechanical compressors is twofold. Firstly there are capacity issues and getting the required pressure rise would be problematic. You would be using a vacuum compressor.
Secondly you are recovering low grade steam using electricity (a lot of electricity) . Common sense would dictate that this is not clever.

Have you ever used any mechanical compressor for compressing of low pressure steam to high pressure. Actually I have never come across any working compressor. ON this forum & others (on the net) I have been searching of the first user who is really using it successfully.
If you can provide the name of such compressor supplier it would be beneficial to all.

Thanks

[Andrei]

Habn,

I do not have the experience of sugar syrup vacuum vaporization, but I have the experience of fruits syrup vacuum vaporization. I guess the processes are not that different. Anyways the pressure/temperature valueas are not that different.

In all your calculations I think you are forgetting something: is there any inert (non-condensable component) at the inlet of your condenser? I think there is, and you should bring that into the equation. Just for water condensation alone you don't need the kind of energy you are mentioning. Either you have an extremely inefficient equipment, maybe designed for other service, or you have a lot of inserts in the feed, caused probably by air ingress upstream of your condenser. Or both of them.

Or, as previous readers noted, you don't give the complete picture of your situation.
Question: where those 3 MW you are mentioning are consumed? Is it at the "finfan" condenser fans motors?

I do not think you have to many options but to look specifically at your equipment, and improve its efficiency. Don't look around too much, you will not find a configuration cheaper than with an air cooler. But look at your air cooler, something is wrong there, shouldn’t consume that much power. Try to design yourself a replacement and compare the results with what’s in your plant.

Andrei

[riven]

Have you ever used any mechanical compressor for compressing of low pressure steam to high pressure. Actually I have never come across any working compressor. ON this forum & others (on the net) I have been searching of the first user who is really using it successfully.
If you can provide the name of such compressor supplier it would be beneficial to all.

What you would use if a vacuum pump train to do that (ie vacuum compressor). Think very basically about a vacuum pump. You are taking a low pressure gas and exhausting it at aa higher pressure i.e. reducing volume i.e. compressor. So I have seen compressors used but they are called vacuum pumps.

What are you trying to do (set up a new thread perhaps)?

[pawan]

Thanks Riven for clarification,
Ya vacuum pumps are OK but they can not be used for large heat recovery purposes.
They are also not really compressors.
I just wanted to have a clarity on the topic.
My objective was to identify if there exist any mech device which can convert LP steam say typically from 3 Atm to any higher level may be 12 Ata or 30 Ata etc.