[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.

Question end================



Answers start =======================
#
The flow rate of the steam can only be determined by the amount of heat absorbed by the slurry. Under perfect insulating conditions that would be the heat required to maintain the steam volume,the steam temp would remain the same but the amount of condensate return would determine the amount of heat exchanged so at 100 deg C, in other words you would have to maintain 1psi of steam at all times so thats your ideal steam pressure to maintain 100degC. So you use a simple f&t type trap and a regulator set at 1 psi. You can also use thermostatic control valves to regulate the steam for a more accurate slurry outlet temp.(the thermostat on the outlet and the control valve on the regulated steam supply.)

Answer 2 start===============   

Oh, goodness! I'm a bit rusty at my ChemE and it's VERY late here (one of the good things about being a Chem E is that you can eventually have your own company, but the hours can be tough sometimes:-), but, I'll do my best to put you on the right road. Also, we used English units and they didn't ram the Metric system down our throats, so you'll have to convert whatever you need to. First, assuming that the steam will enter at 100ºC and leave at 65ºC would only happen if the steam trap being used is a sub-cooling type which complicates the calculation because you have to figure sensible heat AND latent heat, but this problem is not that complicated and you can assume a standard steam trap such as a bucket trap. This, of course, is something they don't teach in Chem E - when you graduate you have VERY little real-world practical knowledge - most ChE's think "packing a pump" is putting it in a box! What you probably should assume is it enters at 100ºC and the condensate leaves at 100ºC - NO sensible heat gain. This would be VERY low grade low pressure steam near 0 PSIG - probably real world would be 30 PSI or greater but if they don't tell you, assume dry saturated steam at 100ºC. It isn't very important, since the enthalpy - latent heat of vaporization value of steam, which is about 900 (at about 60 PSIG) - 970 (at 0 PSIG) Btu/hr does not very greatly with pressure. It is VERY important, however, if you are given a specific pipe diameter for the steam to enter or exit, as the specific volume of steam does vary A LOT with temperature/pressure (remember in most cases you use saturated steam and T & P are tied, as shown by the enthalpy tables). So, assume also that you get condensate leaving at 100ºC therefore the latent heat of vaporization of 970 Btu/hr is the number you need to start with. Google calculator tells me that 12 235 187 (kJ / hr) = 11 596 719.7 btu / hr, so 11,596,720 btu/hr / 970 Btu/lb = 11,955 lbs/hr of dry saturated steam @ 0 PSIG. Of course, the pipe needed to deliver this would be HUGE, that's why you would use higher pressure steam, but the mass flow rate of the steam should not be too different (of course, wait till you have to worry about "overall heat transfer coefficients" and thermal conductivity coefficients...that's where the fun begins... Hope this was of some help. Good luck!

Answer 3 start===============

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). 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.
This is quite a lot of steam for a low pressure system and you are probably going to need a 300 mm diameter steam supply to meet this.
If higher pressure steam is available it may be better to design around a higher steam temperature/pressure.

end.

[Montemayor]

sdlee:

I hope you don’t seriously expect anyone to read all the litany of mumbo-jumbo that you’ve copied into this query – which makes it a variety of opinions, conjectures, gossip, etc., etc.

I’m sorry that I can’t understand your intention in putting your query in such a condition.  However, your basic need seems to be the steam quantity (flow rate) required to heat up a nickel slurry by transferring a given heat load.  That’s easy.

What’s not easy is understanding your communications.  You say you are “heating a nickel slurry with steam at atmospheric conditions from 25 deg C to 63 deg C”.  What, specifically is at atmospheric conditions?  - the nickel slurry or the steam supply?  I think you mean the nickel slurry, right?  Please clear up the conditions of both streams – pressure and temperature.

Your first mistake is a big one.  The worse type of heat exchanger that you could select for heating a slurry is probably a Fixed-Tube-Sheet Shell and tube Heat Exchanger.  This type of exchanger will plug up in no time at all – whether you put the slurry in the tube side - or worse, in the shell side.  The one type of exchanger that has consistent success with slurries is a Spiral type, not a TEMA type of shell & tube design.  The shell & tube design is specifically designed to function only with liquids or gases – never with slurries.  You are going to have to switch exchanger design types if you expect to receive any degree of credibility in your proposal.  Alfa Laval makes Spiral type of exchangers.  You can look them up in the Web.

Your second mistake is a pretty obvious and bad one.  You never, never design a steam-based heater to give up latent heat PLUS sensible heat from the steam source.  (“I was aiming to have the steam enter at 100 deg C and leave at 65 deg C”).  The only heat you should plan to recover from the steam source is the potential Latent Heat of Condensation – period.  The steam condensate you form in the heater (as a result of the condensation) is removed using a conventional steam trap.  The removed steam condensate is saturated liquid and calculated as such.

The flow rate of steam that you need is equal to the flow rate of condensate formed.  Therefore, the amount of steam flow required is equal to the exchanger’s heat duty (Btu/hr) divided by the latent heat of condensation for the steam (Btu/lb) at the conditions inside the exchanger.  That’s why it is so important to know the conditions of the steam you are using.

If you require specific answers, please furnish specific basic data and information.  However, I think I’ve given you all you asked for.  Good Luck.

[sdlee]

Apologies Motemayor for the way in which I described the problem.

Thanks so much for you in depth analysis of my problem, your reasoning and solutions have been greatly helpful.