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Influence Of Steam Pressure On Batch Times

steam batch time

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#1 ThomasH1985

ThomasH1985

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Posted 14 January 2021 - 11:46 AM

Dear colleagues,

 

i have a question and honestly i am feeling a little ashamed for that, but i really do not get it:

 

I am wondering if it makes more sense to run a 7barg or a 3,5barg steam system (both saturated).

 

Of course i already looked in a lot of websites and found out about the advantages and disadvantages of using higher and lower pressures for the steam system:

 

Advantage high pressure steams: smaller diameters, higher temperatures

Advantage low pressure steam: higher specific heat content, less production costs, less flash losses

I also read that the steam traps would become cheaper using low pressure steam, which i acually do not really understand.

 

How ever the most important question for me is if the pressure level has an influence on my batch times (heating up liquids in vessels via jacket pipes):

 

I have a formula to calculate the heat up time based on the steam temperature and and the process liquid:

 

time = C1*LN(t0-t1/(t2-t1)) with C1 = (cpp * mp + cpv * mv) / (k*A);

 

with

cpp the heat capacity of process fluid,

mp the mass of process fluid inside the vessel;

cpv the heat capacity of the vessel and mv the mass of the vessel;

k = heat transfer coeficient;

A = heat transfer area between jacket/coil and vessel;

to = batch start temperature of process fluid 

t1 = steam temperature:

t2 = batch end temperature

 

all with SI Units.

 

In this formula the batch time depends clearly on the steam temperature (t1)

 

How ever looking in the internet around for formulas describing the heat up time of a fluid in a vessel (via jacket or half coils) i only find the formula

 

time = (t2 - t0) * (cpp * mp + cpv * mv)  / (Dh * ms).

 

With Dh = vaporization enthalpie of steam at given pressure (= latent heat during condensation) and

ms = the mass flow of steam.

 

In the latter formula, the batch time decreases with low temperature steam due to the higher latent heat  (assuming the same amount of steam for both pressures; maybe this assumption is wrong?). In the latter formula the temperature of the steam does not play any role.

 

Can anybody help me here? Does higher steam pressure lead to shorter heat up times?

 

BR

 

Thomas


Edited by ThomasH1985, 14 January 2021 - 11:47 AM.


#2 Pilesar

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Posted 15 January 2021 - 02:32 AM

The two formulas are both useful. I think you can use the first formula to solve for time. Then you can use the second formula to solve for the mass flow rate of steam required. For your situation, higher temperature steam condensing will likely give shorter heat up times, but the efficiency would be lower. In other words, there might be some improvement in heat up time with higher pressure condensing but it would probably use more steam to heat the batch. There may be other ways to improve heat up time that should be considered. Internal mixing will help heat transfer. Faster heat transfer might be obtained with an internal coil or forced circulation through an external exchanger. (You don't state the internal temperatures or the process fluid and these are generalities.) Each steam level will have its own network of piping so there is both capital cost and energy efficiency to consider when designing a plant steam system. Heat transfer is best done with steam that does not carry much superheat.



#3 breizh

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Posted 15 January 2021 - 04:46 AM

Hi,

To add to Pilesar's comments , you should consider to perform a cycle time analysis of your process , You may discover room for improvement .I have attached documents about cycle times , calculators to investigate several options for heating up .

Without knowing anything about your products , you may consider to use external HX(s) to heat up the reactants . 

Good luck

Breizh  

Attached Files


Edited by breizh, 15 January 2021 - 05:11 AM.





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