Temperature to energy conversion

[JamesB]

Firstly I will apologies if this is in the wrong place, the question I have falls under thermodynamics (maybe) but I'm working on an engineering project.

Is there a way to determine the time it would take to boil an amount of water when you only have a temperature?  I know you can calculate it if you know the kW/h or BTU used for heating however I do not, I only know the temperature.  Let me explain a little better:

Lets say you have a metal pipe that constantly has gas flowing through it at 400°C.  If this pipe were to go through a tank of water how would you calculate firstly whether you could boil the water, and secondly how long it would take? 

Basically I am trying to work out whether it is possible to use the heat from an exhaust of a burner as a means for heating an evaporator.

The water tank would contain approximately 3500 litres. 

For ease the water starts off at 10°C and needs to heat to 100°C so the delta T is 90.

Thanks for any help.

James

[studiot]

Hello James, and welcome to Science Forums.

Yes this is an engineering question, and I suppose you are an amateur engineer from your description of the problem.

I am a little worried about the safety aspects of handling an exhaust gas at 400^oC so perhaps you would explain in more detail what exactly you are trying to achieve.

Such devices have been made, but the devil is in the detail so

1 hour ago, JamesB said:

for heating an evaporator

Do you really want to boil the water or evaporate it?

Is this clean water or does it contain chemicals or other material that would need residue removal?

What do you want to happen to the evaporated water and is it being replenished as it evaporates??

You have said 3500 litres of water, but given no indication of the quantity or nature of of the exhaust gases.
(You do realise that you can't extract all the energy from the gas as it need some to clear the flue?
Some manufacturers of condensing boilers have been embarrassed by this and other oversights)

[JamesB]

Hi, Thanks for the response.

With regards to safety aspects, if there is anything that does stand out as a potential problem please let me know.  In terms of temperatures, I am constantly working around far higher temperatures than this so there are already a certain amount of safety protocols in place for anything that I will require.

In answer to your questions:

I want to evaporate the water.  I understand that this can be done at lower temperatures than 100°C but I was using it to try and make things easier, basically I just want to know the calculation so that when I do have all of the information I can work out whether or not it is a feasible option.  I'm a chemist/metallurgist but been given an engineering project, as  I hope you can imagine I'm a little out of my comfort zone.

The point of the evaporator is to separate suspended solids from the water, the steam will just go into the atmosphere, so no replenishment required.  As for the chemicals in the water there isn't anything that really raises the boiling point or stop it from evaporating too much.  Mainly various compositions of carbonates and hydroxides. I am aware that there are safety concerns here however, as mentioned above, we do have protocols in place for hazardous chemicals.

As for the quantity and nature of exhaust gas, currently I do not know.  I was hoping that there would be a simple way if you could say the temperature is a constant then you would be able to calculate how long it would take. I know its not 100% efficient but its just a theoretical assumption that I'm looking for.  Just a way to progress with the information I currently have.

I realise that if you know the kW/h then it's fairly easy to calculate, however I am currently working theoretically so we don't know what the burners will be working at.  Hence I was asking that if we can assume a temperature of the pipe we would be able to calculate the time it would take to heat the water to boiling point. It's not the best way to look at it but currently its all I can do.

Hope this answers you're queries and thank you for taking the time to help me with this.  If it is completely impossible to calculate or I sound like an idiot just tell me as much.  It's not really an area I have ever worked in before.

Thanks

James

[studiot]

3 hours ago, JamesB said:

Hope this answers you're queries and thank you for taking the time to help me with this.  If it is completely impossible to calculate or I sound like an idiot just tell me as much.  It's not really an area I have ever worked in before.

 

Good morning, James. It looks as though our timezones clash but never mind we will get there.

Yes lots of very useful fill-in information. Thanks. No you are not an idiot and you have done exactly the right thing by asking.
In fact is is a real pleasure to have a pleasant, intelligent and productive conversation rather than a dispute with some troll.

 

OK let's get the temperature thing cleared up first then see what practical advice can be offered.

Temperature is not a measure of the heat quantity in a body, it just tells you which way heat will flow naturally.
There is a great deal of heat in the ocean, but not easily accessible because the temperature is low.
On the other hand, the temperature of a lighted candle is high, but the heat content is low so you don't burn yourself if you extinguish it by pinching between your fingers.

There were two reasons for my question aboyt the exhaust gases.

I hope you understood the first one about need to leave some energy for the exit kinetic energy of the gas.
As a materials specialist I assume you have covered chemical energetics.

The second reason is to do with the safety aspect.
Fluing requirements (in the UK at least) are very specific to prevent noxious gases from entering the living environment.
There is a substantial minimum flue cross sectional area (aka pipe size) which would not fit well into your tank.
Take the cover off a domestic or commercial boiler and look.
Further, in order to maximise heat transfer you would need thin walled, thermally conductive duct material.
This would contain hot corrosive gases inside and be surrounded by hot water outside and thus difficult to avoid corrosive heat damage.
 

A better arrangement from both the safety and practical point of view might be an indirect heat exchanger system.

Piped water (or other fluid) from a small reservoir is heated within the exhaust duct or by being wrapped around it.
This water is then passed through the water in an evaporator, rather than a tank, inside sealed heat exchanger piping where it heats the water you want evaporated before returning to the small reservoir.

Because the heat exchanger fluid is sealed it can be clean and neither corrode the pipes nor deposit scale within them, preventing degradation of the system.

The evaporator basin itself should be as spread out as possible, allowing maximum surface area to the evaporating water.
It may be beneficial to blow the water vapour away with a fan to assist speed up evaporation.
Electric heating elements can also be deployed in the evaporator to assist if there is not enough energy in the exhaust gas or you are just not running the burner.
Since you presumably wish to recover the solids this would be easier from a flat spread out tray than from a tank.
Cleaning such a tank is much easier.

 

You can get an idea how much heat might be available by multiplying the calorific value of the fuel gas by the usage rate of the burner.
Both will be reasily availble from the manufacturers and should ideally be stated on the equipment.

That is the total heat produced as a time rate, ie per second or per minute. Remember power = rate of doing work = energy per second.

If your system cooled the exhaust from 400^o to say 150^o, then the heat available to your evaporator will be up to (400 - 150) * C_exhaust
where C_exhaust is the specific heat of the exhaust gas and will be readily available from tables.
You do not want to go much below 150^o as the one of the exhaust gases will be water, which you want to keep as steam for exhaust purposes.

 

How are we doing?

[JamesB]

Thanks for the response.  And thank you for the compliment. 

I think I'm getting there.  To be honest it's looking like I'm going to have to get more information regarding the burner itself.

I do have one query regarding your suggestion of using a heat exchanger system, if the liquid inside the heat exchanger is water, this would have a maximum temperature, unless pressurised, of 100°C.  Would this effectively heat up a body of water enough for evaporation to take place in a reasonable amount of time?  increasing the surface area would also speed this up.

I have seen similar systems using exhaust gases from burners before however on nowhere near as large amounts of water.  But like you said, there could be an issue with corrosion of the piping.

I may have to re-think my approach to this.

Thanks for the help

James

[studiot]

13 minutes ago, JamesB said:

I do have one query regarding your suggestion of using a heat exchanger system, if the liquid inside the heat exchanger is water, this would have a maximum temperature, unless pressurised, of 100°C.  Would this effectively heat up a body of water enough for evaporation to take place in a reasonable amount of time?  increasing the surface area would also speed this up.

This is where the engineering decisions and trial calculations come in.

Yes the water could be pressurised to raise the temperature, or the transfer fluid could be steam or oil or a commercial high temperature 'refrigerant' such as you find in heat pumps.

Yes a suitable piping loop could evaporate 3500 litres, but what is the recharge time?

 

That would determine if it was fast enough.