Enthalpy

[Chris]

Moderator,
I am involved with a design problem that requires a small flowstream
(10 lpm) of oil to be heated (or cooled) at least 10 degrees C. This
temperature differential does not have to be permanet. In other
words, the stream can be heated then cooled so that there is no net
change in temperature. To minimize the amount of energy required, I
have considered a refrigeration system that will, for example, use the
condenser side to heat the flowstream and the evaporator side to cool
the flowstream, thereby restoring the temperature.

Since there is no change in temperature to the oil, is it safe to say
that the only energy required is to overcome the inefficiency of the
compressor? Or, have I totally overlooked some basic law of
thermodynamics?

Chris

[Evgenii Rudnyi]

Hi Cris,

So let use say the ambient temperature as well as the temperature of the
stream is To. Then it would be necessary to have temperature To+10 from x1
to x2 in the flowstream and then you would like to cool it back to To from
x2 to x3. Have I understood the problem correctly?

Evgenii

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[Laird, Chris]

Evgenii,
First of all, thank you for taking the time to respond.

It seems that you have the problem right. Maybe the attached diagram
will clarify it somewhat. The simplest way to achieve a 10 degree delta
is with an electric heater. I have calculated that the power required
would be about 3.5 kW. The entire purpose of the refrigeration system
is an attempt at reducing the power requirement. Is this a realistic
goal?

Best regards,
Chris

Hi Cris,

Evgenii

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[Evgenii Rudnyi]

I have to think it over more carefully. The first thought is that the
efficiency of the Carnot cycle

http://en.wikipedia.org/wiki/Carnot_cycle

does not look good when the difference is only 10 degrees. So it is a
question whether it is worth the efforts. Please remember that the
effeciency of the Carnot cycle is the highest efficieny that one could
achieve.

[Laird, Chris]

Evgenii,
Another solution to this problem is to use an electric heater to create
the 10 degree differential then recover some of the heat. After the oil
is heated, it is then passed through a heat exchanger on the upstream
side to preheat the incoming oil. Ignoring loses, this should reduce
the power required by up to 50%, cutting the power to 1.75 kW.

Do you think the Carnot cycle can beat this?

[Evgenii Rudnyi]

So let us consider a heat engine to make a work according to the Carnot
cycle with T_hot 303 K and T_cold 293 K (20 degree Celsius). Its best
efficiency would be

(303-293)/303. = 0.033

Then you can convert work to heat with efficiency 100 percent but it is
still 3.3% from the original difference in heat at hot and cold
temperature. Hence I would say that a heat exchanger sounds to me as a
better idea.

on 20.01.2011 21:24 Laird, Chris said the following:

[Laird, Chris]

Evgenii,
Thank very much for taking the time ponder my thermodynamics problem. Your advice has been very helpful.

Best of luck,
Chris Laird
Pittsburgh, PA
Best regards,
Chris Laird