×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!
  • Students Click Here

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

Jobs

Energy saving per degree increase in temperature

Energy saving per degree increase in temperature

Energy saving per degree increase in temperature

(OP)
Hi,

I am designing a heat recovery system for a milk powder spray drier.  I will be utilising waste exhaust heat (67degrees C 245T/hr) and use it to pre heat the inlet air to the drier.

The drier inlet air is supplyed at 35degees C where it is then heated to 205degrees C by an air heater (160T/hr).  The air is then used in the drier.

I will be putting a heat exchanger in the exhaust and in the inlet prior to the air heater.

I am wanting to do some initial energy calculations to determine viability.  I would like to know the energy saving (and cost saving) that will be obtained for every degree Celcius I pre heat the inlet air before the air heater.  Given an energy cost of $0.12 per kWh.

Can anyone help with these calculations?

 

RE: Energy saving per degree increase in temperature

(OP)
Meant to put up my basic working before.

specific heat of air room temp = 1.012J/g.K

(1 degree C)* (1.012)*160000000
= 161920000J
1kWh = 3600000J
44.98kWh = $4.5
at $0.1 per kWh

There for will be saving $4.5/hr for every degree the inlet air is pre heated.

Are these calculations correct??

RE: Energy saving per degree increase in temperature

(OP)
No worries,

I think I have it right except I was using cost for electrical energy not thermal.

Given the milk powder plant has a co-generation plant on site I am costing according to steam energy prices.

1 tonne of steam = $15NZD
1 tonne of steam = 665kWh

Therefore cost = $0.0232 per kWh
giving a energy saving of $1.04NZD/hr if the inlet temp is increased from 35 to 36 degrees C.

Pretty sure this is correct, hopefully it may help someone else interested.

RE: Energy saving per degree increase in temperature

Engprofile:

I did get a chance to notice that initially something was kind of fishy with some of the units used ...

It is very good that you are reporting your final output values.  In my opinion, it demonstrates that you are serious and do get to the final step of engineering calculations.

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

RE: Energy saving per degree increase in temperature

If you can get your hands on one of the mechanical engineering handbooks, there is a great amount of info. on efficiency, cost saving and payback discussed in combustion topics when air preheaters are installed.  One little fact that you must seriously consider is the dew point of the products of combustion. Cooler products of combustion can cause serious corrosion of the exhaust duct and chimney. You must calculate the dew point temperature to determine if it will be detrimental.  

RE: Energy saving per degree increase in temperature

(OP)
Thanks chicopee,
I managed to get hold of a Mechanical engineering handbook (Energy volume) which has proved very useful.  
I don't however understand how the dew point will effect the heat recovery process.  Because I am only preheating the air before the air heater.  There are no combustion products in the exhaust stream.
Am I miss understanding something?

RE: Energy saving per degree increase in temperature

The air is preheated with the product of combustion (POC) indirectly.   As the POC temperature drops on the oulet side of the air preheater, the dew point temperature gets to be closer to the POC temperature.  Any cold surfaces(chimney,ducts, plenum) whose temperature is at or below the dew point temperature of the POC will condense the moisture ladden POC and corrode metal and mortar. Also realize that the condensation is a mild carbonic and sulfuric acid.

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!


Resources