Fixed-Tube-Sheet Shell and tube Heat Exchanger

[sdlee]

Hi guys,

I've been on a few message boards looking for a solution to my problem, but no one has really been able to help me yet. Here is the problem:

Question===================
Hi all,

this is my first post/thread, just starting out as a second year chemical engineer and I seem to have found myself in need of some help.

Basically, I'm designing (basic) a heat exchanger where I'm heating a nickel slurry with steam at atmospheric conditions from 25 deg C to 63 deg C.

I have the correct heat duty of 12235187 kJ/hr through using Q=m*Cp*∆T

m = 4165.19 kmol/hr * 77.302 kJ/K.kmol (avg cp)

Now that I have the heat duty that is required to heat up the nickel slurry, I am unable to understand how to get the mass flow rate of the steam required to do this.

I was aiming to have the steam enter at 100 deg C and leave at 65 deg C, is this reasonable? How do I work back from the heat duty required using the temperatures mentioned to work out the mass flow rate of the steam?

I know that I need to take into consideration condensation and other factors, but I'm just not sure what to do.

Any help would be appreciated.



Additional Details


Thanks so much for your help. I'm not too sure yet what the steam pipe diameter will be, but I'm designing a fixed-tube-sheet 1 shell 2 pass HE.

If you were to give your expert opinion, what would have the steam coming in at pressure-wise?

I have assumed that the nickel slurry is moving through the HE at atmospheric conditions.

Assuming that your steam supply is entering the heat exchanger dry saturated and exhausting to an atmospheric condenser (ie the steam is at least 100 deg C) the heat extracted from the condensing steam will be 2,256 kJ/kg (from steam tables).

Thanks so much for your help Ynot, but why do we use the evaporation value from the steam tables instead of the steam?

You can ignore any further heat gain in dropping the condensate temperature to off set against any thermal losses.
A heat load of 12,235,187 kJ/hr will require
12,235,187 / 2,235 = 5,474 kJ/hr steam, or 5.5 tonnes/hour.


If higher pressure steam is available it may be better to design around a higher steam temperature/pressure.

I'm planning on using a fixed-tube-sheet exchanger with the slurry tube-side and the steam shell-side. If the slurry is at atm pressure, what would you recommend as a good operating pressure for the steam to reduce the size of the shell?

thanks again, steve.

 

[Astronuc]

It'll depend on the heat exchanger design and whether or not it is parallel or counter flow.

basically one needs two equations, one for each flow, and the heat transfer rate is related to the mass flow rate and change in specific enthalpy, e.g. $\dot{m}c_p\Delta{T}$ for each flow.

 

[ravenprp]

Hi Steve,

I understand your frustration with not being able to find a solution to your problem. Designing a heat exchanger can be a complex task, especially when it comes to determining the mass flow rate of the steam required.

From your calculation, you have correctly determined the heat duty required to heat up the nickel slurry. To determine the mass flow rate of the steam, you need to consider the heat transfer coefficient and the temperature difference between the steam and the slurry. These factors will affect the amount of heat transfer that can occur.

As for the steam pipe diameter, it would depend on the design of your heat exchanger and the required flow rate of the steam. I would recommend consulting with a mechanical engineer or using a heat exchanger design software to determine the appropriate diameter.

In terms of pressure, it would be best to use the highest pressure available for the steam to reduce the size of the shell. However, this should also be balanced with the cost and availability of higher pressure steam.

I hope this helps and good luck with your design!