Flash Steam Heat Recovery 3

[lucino]

Good Day!
I am presently a part of an Operations Improvement Team.
In one of my studies, I found out that steam is flashing out from a condensate recovery tank with an estimated 756 kg/hr incoming condensate rate at 84oC water temperature.
I also found out that nearby the condensate recovery tank, a compressed air at 5 bar is heated by an electric coil rated 6kW in a heat exchanger with a chamber temperature of 104oC for atomizing syrup spray.

I planned to suggest that the flash steam will replace the coil..thus savings in electricity will be realized.

Is this idea technically possible? How to prove it by engineering calculations?
Thanks.

 

[25362]

Are you sure steam is flashing out of condensate at 84[sup]o[/sup]C at atmospheric pressure ?

 

[quark]

A flash steam recovery system is unnecessary in your case. Better thing you can do is using hot condensate to heat compressed air. By this way you can reduce electric power input.

As 25362 said flashing at atmospheric pressure is not possible and vacuum flashing is a wastage in your case. As your air requirement seems to be quite low (approx. around 2.5 cfm, considering heating of air from 30 deg.C to 104 deg.C) You have to check for installation cost vis-a-vis savings.

A rough estimate of maximum savings can be 3 kW. (because you have to use electric heaters to raise the temperature after an intermediate temperature)

I would prefer a payback period of 18 Months to 24 Months at the maximum.

Always keep the system simple.

Regards,

 

[quark]

I am sorry. There is an error in conversion. Is the actual air flow 140 cfm?

But the savings will be same as I stated earlier.

Regards,

 

[lucino]

Hi 25362.
It is not steam flashing at 84oC but this is the temperature of the incoming condensate to the condensate recovery tank which is flashing steam to atmosphere.
Thanks for the comments.

 

[lucino]

Thanks Quark.
Your comments is too helpful.
Please show however calculations how you arrived at the approximate 3kW savings.
Sorry I am an electrical engineer and I am not too well familiar on heat transfer calculations.
Regards.

 

[quark]

Lucino!

First of all water evaporates to steam at 100 deg.C at atmospheric pressure. So at 83 deg.C and at atmospheric pressure there is no chance of flashing condensate.

Assuming your 6 kW heating load the calculation goes like this.

6 kW x 3412.14 = 1.08 x cfm x (T2-T1)
Where 3412.14 is conversion from kW to Btu/Hr
T2 = Final temperature of air in deg.F
T1 = Initial temperature of air in deg.F

Considering 30deg.C initial temperature
6 x 3412.14 = 1.08 x cfm x (219.2 - 86)
Therefore cfm = 142.31

Now with 84 deg.C water temperature, you can economically heat the air upto 75 deg.C (it is an estimation), then after you have to use electric heaters.

So electric power required is
P = 1.08 x 142.31 x (219.2 - 167) = 8022.86 Btu/Hr = 2.35 kW

I will consider it to be 3 kW to account for all inefficiencies. So at the most, you will have 3 kW savings.
Say your plant runs for 10 hrs a day. i.e. 30 kW-Hr

For 313 days a year (excluding Sundays) it is 9390 kW-Hr.
When compared to my monetary units and cost of heat exchanger, the payback period is more than 2 years.
(Equivalent to 0.08 USD/kW-Hr)

If your energy cost is high, then best of luck.

Regrds,

 

[lucino]

Dear Quark,
Thank you.
Your comments are most helpful.
The calculation is very well presented.
I can use this as pattern to other heat transfer savings initiative.
More power

Lucino

 

[chicopee]

The effect of steam to atmosphere from the condensate tank appears to be deaerated vapor from the tank just like vapor from your cup of coffee and this vapor is not flashing steam.

 

[lucino]

Good day Chicopee.
I think it is.

But I also suspect that one of the steam traps is leaking introducing steam to the condensate tank which in turn leaks to the atmosphere.
By this my problem is how to identify this leaking trap because all the condensate lines (about 6 pipes)towards the tank has no drain valves as well as all steam traps.I also observed that steam traps are not installed according to standard piping lay-out.

Any comment???
Regards.

Lucino


Any idea an

 

[TBP]

Make sure that all traps are 1/ the right style 2/ the right size, and 3/ installed correctly. I've seen a number of heat recovery projects that came to nothing because the "waste" heat that was deemed possible to recover, was in fact only available due to some other fault. That fault often gets identified and corrected during the course of the project, making the initial project a waste. That's always disappointing.

Even in those cases where it is possible to utilize flash steam, be aware that it only exists due to something else running. If that "something else" is out of service for some reason, and you've come to rely on it, you're out of luck. That's what often kills recovery projects - it's just too awkward to co-ordinate production runs and maintenance outages.

 

[lucino]

TBP,
Good day.
Your comments are well taken.
Actually this also a line of thinking I am at the moment considering.
I agree with you that recovery projects should be sustainable.
Thanks for the input.

Lucino