Electrical Heaters for Process Unit

[sav455]

Hi All,

I have a heat exchanger which heats a small stream (referred to as slip stream, flow not known)to 250 degC using 4800 kPag satrated steam. The amount of steam for this heater is 159 kg/h. The heat duty as given in heat balance is 0.26 Gj/h.
I was just wondering if it makes sense to replace this heater with an electrical heater. Is is practical?

Ragards.

 

[quark]

Check the enthalpy of steam at the process condition and get the total power required (it is actually 69% more than your heat balance value). Compare running cost of steam and electricity.

Is it free steam or your boiler is specifically installed for this application? Why the comparison was not done during installation of this high pressure system? Is this a homework problem?

 

[sav455]

Thanks quark.
This is not a homework problem.I think I did not define it clearly. My first post was ......bad.
I am doing this comparision at the installation stage.The steam is taken from a steam generation unit which is already existing. This steam is to be supplied to the heater as mentioned in my first post.The amount of steam is 156 and not 159 kg/h at 4230 kPag (375 degC, superheated)(sorry).
The amount of steam is so small that to my understanding it does'nt make sense to install a 1 or 1.5" 100m line.So, I thought of changing the steam heater to electrical heater.
A quick cost estimate indicated that electrical heater will be more expensive (~ 1.2 times) to operate than the steam heater.
0.26 Gj/h is ~ 72kW.So, this will be the size of electrical heater. I have no idea how efficient are the electrical heaters. This figure (i.e. 72 kW) may turn out to be slightly lower if they are more eficient than the steam heaters.

Regards.

 

[25362]

I've had satisafactory results with electric process heaters. Suggest you contact with a maker of these.One link could be:

http://www.marmstrong.com/products2.htm

 

[sav455]

Thanks 25362.

 

[quark]

Your calculations are close to what I got. The enthalpy of steam at 43.3bara and 375⁰C is 2869.7 kJ/kg and if your condensate is saturated at 43.3bara then the enthalpy is 1110.72 kJ/kg. So total heat outflow will be 156*(2869.7-1110.72) = 274407.12 kJ/hr or 76.22 kW. You can safely assume electrical heaters as 100% efficient.

If 76.22 kW is the heat input the process fluid requires, you can't reduce it. It is better to do a heat balance of heating and process fluids.

What is the cost of 1kW-hr at your place?

 

[djack77494]

The efficiency of an electric heater can be taken as (approximately) 100%. It's very easy to convert high quality energy (e.g. electricity) into much lower quality energy (heat), hence the high efficiency. Steam heating will also be very efficient, though not quite that good.

In general, I would expect the operating cost for steam heating to be quite a bit lower than those of electric heating, simply because electricity is the highest quality and therefore often the highest cost of your various available energy sources.

Looking at the power requirement calculated by quark (76Kw), I would guess that you're in the size range where steam heating would be the better choice. (But don't take my word for it, do the calcs.)
Doug

 

[sav455]

Thanks Quark and Doug.
The cost of one 1 MWh = 45 $
HP steam = 17.76 $/tonne
Both are canadian dollars.
Regards.

 

[quark]

Yes, steam seems to be cheaper in your case.

For example, considering 76.22 kW-hr, the electricity cost would be 76.22*45/1000 = 3.4299$. The steam consumption of 76.22kw is 156*17.76/1000 = 2.77$.

Condensate and flash steam recovery may further reduce the steam cost.

djack hit the nail about the quality of energy. However, when you take installation cost into consideration, electricity becomes cheaper sometimes (this is not true if you are using an surplus existing steam generation system)

 

[sav455]

Thanks Quark.