Energy saving per degree increase in temperature

[Engprofile]

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?

 

[FOURe]

How about if you check out the following URL -- EngineeringPage.com:

http://www.engineeringpage.com

Go under heat exchangers ...

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

 

[Engprofile]

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??

 

[Engprofile]

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.

 

[FOURe]

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

 

[chicopee]

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.

 

[Engprofile]

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?

 

[chicopee]

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.