calculating steam flow

[oil198]

Hi,

I have a vessel which is half-filled with water. They want to use steam to heat it up at a rate of 20 deg C per hour. How do I calculate the the steam flow in kg/hr for the required heat increase?

 

[FOURe]

I would suggest that you do an energy balance for incoming and outcoming streams -- inlet and outlet conditions need to be specified (temperature and pressure values as well as the enthalpy values) before one can see what the mass flow rates are ...

http://www.engineering-4e.com

 

[oil198]

yes i can do the energy balance actually i think what was throwing me off was the water in the vessel...its been a long time since i did this calculation but i can calculate the energy needed to heat the steam by first determining the mass of water in the vessel when its half full...after calculating Q, the steam mass flow rate can be calculated..

 

[FOURe]

My suggestion is to specify the inlet and outlet conditions first in order to end up with the stream mass flow rates ...

Once you have your first case done, you can always go back and give another try with some other conditions, if required ...

http://www.engineering-4e.com

 

[oil198]

no energy balance in the sense of input and output streams is required...i was just trying to calculate the amount of direct heat injection into a vessel, it was just a simple
Q = m Cp delta T...once you calculate the energy required divide by the total enthalpy in the steam at the supply pressure...

 

[FOURe]

Yes, you are correct for the vessel water and/or heat input requiremenrs.

For the steam side, you do have one stream coming in and one coming out. Inlet steam conditions need to be specified. The outlet conditions will come out form the energy balance calculations - constraint that needs to be satisfied.

That should do it ...

http://www.engineering-4e.com

 

[HEC]

oil198
Altenative approach is to consider the problem as heat flow or kW.
first determine kW required- Q = m Cp deltaT (deltaT is 20). This also neglects heat loss. a small allowance needs to be made for this and will be determined by tank dimensions, insulation etc. The heat will provide Joules input required divided by the time over which this is needed (1 Hour). Divide the Joules input by time (3600 seconds) will provide the average kW (kJ per second) flow required.
From given steam conditions determine the joules output for condensing 1 kg to the final water temperature required. This will provide a heat mass flow kJ per kg steam.
It is then a matter of dividing kW by kJ/kg to get kg/second of steam!

Mark Hutton

 

[FOURe]

HEC:

That should do it ...

http://www.engineering-4e.com