Steam Consumption of Heat Exchangers

[ergungungor]

Hello Everyone,

Before I start my question, first I'd like to apologize for any illogical thing, I may say. It's been so long since I've dealt with thermodynamics and even back then I wasn't very good at it.

I'm trying to calculate the amount of steam consumed in a heat-exchanger while heating up a certain amount of water that will then be used in processes. Eventually, I'd like to reach a value that would indicate increase in temperature/unit cost (i.e. increasing the temperature 2 degrees costs 1 dollar). My main limitation is the information about the steam network and boiler. System is set to supply steam at 66 psi and the only other thing is the furnace temperature which is around 300 F. Although, I've some information about how much natural gas is used for the boiler.

Considering the limitations, I'm not seeking a terribly accurate result. I've very simply tried to work my way through Q=mcdt and tried to calculate how much energy would be needed to heat up a given "m" amount of water. I thought If I can calculate the amount of steam that would be used to heat up that given amount to a certain degree, I could reach to a cost value from there. After investigating a bit, I found out that I can use the equation mcdt=Q=hf*ms(in which ms is the Steam consumption(kg/s)and hfg = Specific enthalpy of evaporation (kJ/kg). First, I used the total amount of water instead of the flow and the cost i got in the end seemed very wrong. (i.e. to increase the temperature of 378 L water by 54 degrees, it costs $68.) So I used the flow rates as suggested in the formula and It still didn't make any sense. Also this equation does't take any consideration about the current steam system. While I don't have much info about it, it still seems like a missing piece. I mostly tried to use the calculations from this website :

http://www.spiraxsarco.com/resource...sfer/steam-consumption-of-heat-exchangers.asp

In short, my question comes down to; what is the amount of steam required (to be used in a heat exchanger), to heat up X amount of water, Y degrees.

If anyone has any helpful ideas, I'd be happy to hear it.

Thanks you for spending your time and reading.

Best Regards,
Ergun

 

[ione]

Ergun,

It seems you have applied the right formulas, maybe not the right units.

First you have to compute energy required to heat up water : Q = m*c*deltaT

m (mass of water to be heated) expressed in kg or lb
c (specific heat of water) expressed in kJ/(kg*°C) or Btu/(lb*°F)
delta T (temperature increase required) expressed in °C or in °F

Then you use the second formula: Q = ms*hf

ms (mass of steam required) expressed in kg or lb
hf (specific enthalpy of evaporation at your pressure) expressed in kJ/kg or Btu/lb.

In your specific case, assuming you have available saturated steam at 66 barg pressure the value of specific enthalpy of evaporation is approx 2095 kJ/kg or 901 Btu/lb.

Just factor in time to deal with power instead of energy and get mass flow rate instead of mass.

What above assumes the heat transfer process has an efficiency of 100%, which is not the case, so apply an opportune margin.

 

[ione]

Ergun,

66 barg was a clear mistake, I did mean 66 psig.

 

[willard3]

You should read forum policies which begin as follows:

>>>>>>These fora should not be used to bypass your own in-depth research on the issues that affect you, nor is it intended to be a substitute for appropriate professional assistance within your field or geographical region.


Hire an Engineer who does this and don't try to bootstrap this

 

[ione]

Priceless as usual

 

[ergungungor]

Dear Ione,

Thank you very much for your reply. I'll go through the calculations once more and see how it goes.

I tried to avoid factoring in the time, because the water is not used right after the heat exchanger but it goes to a storage tank. So I actually don't know the hydraulic retention time (or whatever it is called in HE literature)since both the water flow rate and volume of the tank are currently unknown variables. I got the flow rate value after the storage tank but it's not necessarily the same rate before the heat exchanger.

I'll try to find some more information and probably make some assumptions.

Once again thank you very much for your help.

Dear Willard3,

Thank you for the reminder. I'm not bypassing my own research in anyways, my research is a lot more wider than that and this was just an area that I'm clearly not an expert on. I've actually tried several ways and researched for a while before asking here and I just thought I'll give it a try and maybe someone will have suggestions. If you actually read again, I asked for ideas or suggestions, didn't give away all the data and ask someone to do the calculations for me. I'm interested in learning the method and ways rather than buying the results, so I can apply it to other problems in the future and If you're going to restrict sharing knowledge and information what's the meaning of using forums like this. Plus, last time I checked they wouldn't allow/pay PhD students to hire consultants and/or engineers.

Sorry If that was disturbing for you and you had to read it. Hopefully you can avoid future comments until an admin deletes the post(considering it's really violating the forum rules.)

Kind Regards,
Ergun

 

[ergungungor]

Sorry, I meant to say volume of the heat exchanger instead of volume of the tank.

 

[chicopee]

There are other considerations beside applying formulae such as fouling, therefore, you would be better off to contact a HX manufacturer or representative to help you out. .