Jump to content

  • Log in with Facebook Log In with Google      Sign In   
  • Create Account


Featured Articles

Check out the latest featured articles.

File Library

Check out the latest downloads available in the File Library.

New Article

Product Viscosity vs. Shear

Featured File

3-Stage Propane Ref Performance

1
- - - - -

Estimating Heat-Up Time For Product Tank

tanks heating start-up time coils steam hot water calculations

4 replies to this topic
Share this topic:
| More

#1 Frankdj

Frankdj

    Junior Member

  • Members
  • 10 posts

Posted 20 March 2017 - 10:27 AM

After calculating the KJ required to heat the steel of a tank, say Q1,  plus the KJ to heat up the product, say Q2,  and knowing the rate of loss of heat once up to target temperature, say P3 ( and estimating that during the start-up that the rate of loss of heat is 50% of the full rate,  due to the temperature gradient during that phase ).   We can use (Q1+Q2) / (Latent heat of the steam)   = part of the mass of steam required, enabling one to attempt a first pass estimate of the time for the start-up, and then apply that time to convert P3 to KJ also,  to then add to Q1  and Q2 for an approx total KJ .......  it would be nice if it were simple and just a function of the steam flow rate.  But it is compounded by a heat balance equation,   and the unknown loss of heat of the ejected steam / condensate mix  plus unknowns such as the product ( bulk liquid ) changing properties during the heat up, eg  viscosity, density, thermal conductivity, etc.   It seems that one has to resort to empirical data for estimating a start-up duration.  Any  advice or comments are welcome.  There are no specific bulk liquids in question,  This is a general issue, in particular for heating coils inside tanks,  with steam or even hot water as the heat source. Thank you.    

Regards  Frank  



#2 Frankdj

Frankdj

    Junior Member

  • Members
  • 10 posts

Posted 20 March 2017 - 06:15 PM

Please note, in the above case Natural Convection is envisaged for the product, not forced circulation.  
Thanks
Frank


#3 sgkim

sgkim

    Gold Member

  • ChE Plus Subscriber
  • 240 posts

Posted 21 March 2017 - 04:33 AM   Best Answer

Hi Frank,

 

To calculate the heating up time easily, you'd rather simplify the system by assuming the following system parameters be constants:

 

  steam temperature (ts), overall heat transfer coefficient(U), surface convection loss coefficient,(h), specific heats of product and steel(Cp & Cps), ta=air temperature,

  m = mass of product,   ms=mass of steel, Ac=surface area of coil (or jacket), Av=surface area of vessel  

 

Heating duty can be supplied by the heat transfer in the system deducting the heat loss to atmosphere due to natural convection on the surface Av. 

Heat balance:  (m*Cp + ms*Cps)*dt/dθ  =  U*Ac*(ts-t) - h*Av*(t-ta)  .........(1)  <= the only variables are time, θ, and product temperature, t.

 

You can calculate the time required θ by integrating the equation (1) from initial temperature t1 to final temperature t2 :

θ =(integrate from 0 to θ) ∫dθ = (integrate from t1 to t2) ∫ {(m*Cp + ms*Cps) dt / [U*Ac*(ts-t) - h*Av*(t-ta)] } .........(2)

 

You can neglect the heat loss term, h*Av*(t-ta), if the surface of the vessel is well insulated to minimize the heat loss. 

 

~Stefano


Edited by sgkim, 21 March 2017 - 04:37 AM.


#4 Frankdj

Frankdj

    Junior Member

  • Members
  • 10 posts

Posted 21 March 2017 - 12:26 PM

Hi Stefano,

 

Your response is fantastic.  Problem solved .   Thank you very much.

 

Kind regards,

 

Frank



#5 sgkim

sgkim

    Gold Member

  • ChE Plus Subscriber
  • 240 posts

Posted 23 March 2017 - 02:01 AM

Frank, 

 

The total steam requirement for the whole heating sequence and the instant steam requirement at time t are not yet answered by me.  Evaluate these data from the equations given in my previous posting.

 

~Stefano






Reply to this topic



  

Similar Topics