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Steam Requirement for Textile Dyeing Machine

02/20/2016 1:29 AM

Hi All, We are considering buying dyeing machines kiers for batch dyeing cotton/synthetic made fabrics. We have two option both are pressurised type vessel which can go upto 135 C which is a requirement for polyester dyeing. But one machine has an additional option that in it after 85C if you want to raise the temperature it seals the machine and takes in compressed air @ 2.5 to 3 bar pressure. This machine manufacturer claims that in this way if you raise the temperature to 135C the boiling point is increased and you save a lot of steam (heating is done indirectly through a shell and tube SS316L heatexchanger using steam). Can someone give the calculations for how much steam would be saved in the machine that fills itself with compressed air as compared to the other machine in which the machine is sealed but no compressed air is filled in if we have 10 KG water inside the vessel and we want to raise temperature to 135 c from 30C.
Thank you

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#1

Re: Steam requirement for textile dyeing machine

02/20/2016 6:34 AM
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#2
In reply to #1

Re: Steam requirement for textile dyeing machine

02/20/2016 6:52 AM

Thank u.

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#3

Re: Steam Requirement for Textile Dyeing Machine

02/22/2016 12:34 AM

I am trying to understand how you heat the water to 135C in the system without the air.

I think you are saying the system is sealed and so the act of boiling the 10kg water and generating steam which then has nowhere to go will increase the pressure. There will also be air in the dyeing machine (I expect) and heating this without increasing volume will increase the pressure in the system.

Generally for the temperatures you are considering there is not much change in the heat capacity of liquid water (and dye and textile) from ambient to 135C so for both systems heating the liquid water and fabric will be about equal.

So I think the saving comes because if air is used to pressure the system then no water is vaporised to generate pressure. Vaporising water is extremely heavy on energy (2.2MJ/kg approx at the conditions at which you are working).

The compressed air system takes the pressure of the system above 2.2barg (the pressure at which water boils at 135C) with a margin so that all 10kg of water will stay as hot water and so steam is only used for the sensible heating of the water and the fabric.

For the other system which must be sealed otherwise as the temp tries to increase above 100C if there is no increase in pressure then the water will boil at 100C. It is a difficult calculation as it is necessary to know total volume of the system and the volume of air in the charge. Heating from 85C to 135C (360K to 410K) is approx 15% increase in absolute temperature and so might see a 15% increase in pressure ie from atmospheric to 0.15barg so most of the increase in pressure is due to vaporising water. To determine how much steam estimate 1.8kg/m3 for 2.2barg steam and use the free vapour volume.

You have not specified the steam pressure for the heating steam but unless it is much higher than 10barg the latent heat of this heating steam will be 2 - 2.1 MJ/kg ie less than 10% lower than for vaporising water in the dyeing machine. So if your machine has 10kg water and you vaporised it all that would take 11kg of heating steam. This seems like a small amount but if you are processing hundreds of batches it might be significant.

A key parameter will be the vapour space in the machine. As I say above this space is pressured by vapourising water. Clearly the smaller the air space in the machine the less water will be required to be vapourised. Ignoring the air initially in the space a 1m3 vapour space would require 1.8kg of steam and so 2kg usage of heating steam MORE than the compressed air system. Again the benefit of this saving depends on the number of batches processed. As there is only 10kg of water I suspect this errs on the high side it would be inefficient to have 1m3 of vapour space which is useless for dyeing fabric when the water only occupies 0.01m3 so the difference between the two systems seems vanishingly small

In terms of control I would intuitively (and so therefore it could easily be wrong) think that the compressed air system would be easier to control because there is no change of phase.

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#4
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Re: Steam Requirement for Textile Dyeing Machine

02/22/2016 2:52 PM

Thank you for the detailed reply. This is the exact information that I wanted to get to. The 10 KG figure I gave was just for ease of calculations. Actually the dyeing kier actually has a capacity of 1200 KG fabric and for each Kg of Fabric a minimum of 4 Kgs of water are required. Also we plan to do 2.5 batches per day so in a year there should be well over 700 batches of dyed fabric. The cost of the dyeing machine with 5 bar working pressure is significantly higher than the regular 2.5 bar working pressure machine without compressed air filling system.

If I have 1200 Kg fabric and I want to raise temperature from 30 to 135 in conventional non compressed air filling machine my calculations are as follows:

1) from 30c -100C = 4800 KG water X 2.201X 70 = 739536 btu

2) from 100C- 100C vaporisation = 4800 X 539 Kcal X 3.96 = 10245312 btu

3) from 100C to 135 = 4800 X 2.201 X 35 = 369768 btu

In the machine filled with compressed air the 10245312 btu will not be required as the boiling point will be greated than 135C and we would save 10245312X3.95/539= 4800 Kgs of steam per batch

Please verify if this calculation is correct

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#5
In reply to #4

Re: Steam Requirement for Textile Dyeing Machine

02/22/2016 8:17 PM

No it is NOT correct

Please think about the process. You have vaporised ALL of the water in the dyeing kier. That is NOT necessary. As I said in my previous reply you need to find out how much free space is present in the kier and vapourise enough water to raise the pressure to 3barg.

Your final arrangement has fabric tumbling in a steam bath does this seem like a sensible arrangement to you?

There is a worrying statement that a minimum of 4kg water per kg fabric is required. What is the maximum? Is it 4.4kg, 5kg 20kg per kg of fabric this is a critical parameter in determining the volume of the kier.

Say 4800kg of water. At 135C this is 930kg/m3 so 5.2m3

1200kg of fabric of (I think 1400kg/m3) so 0.9m3

This gives a charge volume at 135C of 6.1m3 (5.7m3 based on 1000kg/m3 at filling temperature)

As a guess the volume of the Kier is 6.5m3

so 0.4m3 of volume is occupied by steam at 2.2 barg 135C with density = 1.75kg/m3

So you need to vaporise 0.4 * 1.75 = 0.7kg of water

SO the saving is quite small. (note for simplicity I have assumed that the space above the charge is empty - in practice there is air and water vapour which will reduce the amount of water to be vapourised but only by a small amount)

For you energy calculation it is fraught with danger to mix units in the middle of calculations I would recommend either use metric or Imperial / US Customary and convert at the end

Cp Water over 30 - 135 C avg approx 4.1kJ/kgK = 4800 * 105 * 4.1 = 2,066,400 kJ

Latent Heat = 2200 kJ/ kg * 0.7 = 1540 kJ

You have omitted the heating of the fabric which will add maybe 10% to the load

Total (excluding fabric) = 2,068,000 kJ

= 1,960,000 BTU

The main error in your calculation is Specific Heat = 2.201 BTU / kg C

The BTU is defined as the heat to raise 1lb of water by 1F

So this becomes 2.201 BTU / kg F (2.201 lb =1kg)

And 3.96 BTU / kg C (for temp difference 1C = 1.8F)

And finally 4.2kJ / kg C (1kJ = 0.9478 BTU)

This is why I said that you should use a coherent unit set for the calculation and convert at the end.

I am worried about this calc as I feel I may have missed something important because this means that the difference between the two systems (ie the energy for vaporisation) is less than 0.1% but it wouldn't be the first time that a sales person has not understood the engineering principles of something that they are selling.

As I said previously I think that the compressed air machine would give tighter temperature control.

For the vaporisation to have a significant impact the volume of the kier would need to be >100m3 or to require the pressure raising >10 bar both of which are unlikely

So I think that there is negligible difference between the two systems in terms of the energy consumption but please review my calculations they have been done in a hurry.

Also please think about the process it should have been clear that vapourising all of the water was wrong. Engineering is about know which numbers to combine and seeing how a process works. That is why I am not sure about my answer I feel I am missing something.

Good luck

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#6
In reply to #5

Re: Steam Requirement for Textile Dyeing Machine

02/22/2016 8:22 PM

It appears that the OP wants to conduct a reaction at a certain temperature, at a certain pressure and a certain relative humidity to drive the reaction and not have liquid water present below saturation.

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#7
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Re: Steam Requirement for Textile Dyeing Machine

02/22/2016 8:39 PM

Perhaps but I don't think so.

I am no expert on the dyeing process but the digging I have done suggests that liquid water is required.

Also unless some of the vaporised water is vented the system will explode. The OP has 4800kg of steam in a space of 5.6m3 ie 860kg/m3. Even at 220barg steam does not have this density so the system would have a problem.

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#8
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Re: Steam Requirement for Textile Dyeing Machine

02/22/2016 8:57 PM

There is data missing. What is the void ratio of the fabric. Solid wool or cotton will have a density lower than water.

At a guess the density will be about .2

Here is a good reference

I think the OP should read this and follow some of the references.

Ae are shadow boxing

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