heating and cooling and water

[tangir1]

It is going to hit 80's F this weekend, and time for some of us to replumb chiller if we took it off-line this past winter!

I would like to use this thread to talk about heating/cooling water and it relationship to energy, kind of a FAQ, if you will... ( Can some kind folk check my math for me?)

0, Common conversions:
a, 1C = 9/5 F
b, 1HP = 745 W
c, 1 W*hr = 0.859 kcal


1, How much energy does it take to heat one gallon of water by one degree (F)?

It takes one calorie to heat one gram of water one degree Celsius.

There are 9/5 F per Celsius, and there are 3785 gram (~8lb) per gallon, so it is about 2100 calorie, or about 2kcal per gallon*F

2, How much heat does one gallon of evaporative cooling removes?

Latent heat of water is 540 cal/gram, so one gallon of evaporative cooling is 3758*540 = 2029 kcal.

3, Exercises:
a, How many degree (F) pull down (assume perfect thermodynamic efficiency, etc.), can one expect when running a 200W (~1/4 HP) chiller for one hour?

b, How many gallon of water loss by evaporative cooling is equivalent to running a 200W chiller for one hour?

See who can do the exercises first...

 

[Acrylics]

3, Exercises:
a, How many degree (F) pull down (assume perfect thermodynamic efficiency, etc.), can one expect when running a 200W (~1/4 HP) chiller for one hour?

b, How many gallon of water loss by evaporative cooling is equivalent to running a 200W chiller for one hour?

See who can do the exercises first...

Ok ck, I'll bite, but first define the questions better.

The amount of pull down is also affected by ambient air temp and water temp. So these must be defined prior to specifically answering the question. Ie., if water temp is 33 F, the chiller would be hard pressed to pull it down much at all.
If tank temp is 85, the chiller will have a much easier time pulling down if the ambient air temp is 50 rather than 90. So tank and ambient temp do have this impact.
Also, the energy efficiency of a chiller should not be referred in watts or HP, but rather in BTUs which is a far more accurate method of determining a chillers true potential.
If you really want to get into it, is the tank made of glass or acrylic or ? what thickness is it? does it have foam under the tank or open? All of these and more are part of the overall insulative of the tank and will affect the efficiency of the chiller to do it's job. hee hee
So, until these are clarified, I will consider them trick questions and unanswerable.

James

 

[hdtran]

I think tangir1 is asking whether 'tis better to cool evaporatively or better to cool via a chiller.

We also get into the questions of whether the chiller is a Peltier device chiller (probably unlikely) or a refrigerative 'freon' cycle chiller.

While the latent heat of evaporation of water is important, you also have an issue with relative humidity and ambient temperature of the room, which drive the evaporative rate from the tank.

James, I agree that BTU/hr is a useful unit, but as an engineer, I absolutely despise BTU's, and advocate SI units. The calorie to joule conversion is about 4 J/cal (to 1 significant figure). So, it takes about 4 J to heat 1 gram of water by 1 deg C. The BTU is the amount of energy needed to heat 1 lb of water by 1 deg F, if you insist on using BTU's.

For what it's worth, 1 BTU is about 1 kJ (to 1 significant figure). So, a 200W chiller (chilling at a 200W rate, so consuming probably 1 kW of electricity) chills at about 0.2 BTU/sec.

I leave tangir1 to convert BTU/sec to BTU/hr.

 

[Acrylics]

I think tangir1 is asking whether 'tis better to cool evaporatively or better to cool via a chiller.

We also get into the questions of whether the chiller is a Peltier device chiller (probably unlikely) or a refrigerative 'freon' cycle chiller.

While the latent heat of evaporation of water is important, you also have an issue with relative humidity and ambient temperature of the room, which drive the evaporative rate from the tank.

James, I agree that BTU/hr is a useful unit, but as an engineer, I absolutely despise BTU's, and advocate SI units.

While I understand your feeling, my intent was to get rid of the hp rating and replace with something more accurate. Even watts don't necessarily mean much insofar as efficiency goes.

Furthermore, we need the volume of the tank as well

James

 

[tangir1]

>... but first define the questions better. ...

Alright, alright! Geez, tough people here

I admit that the first question is not well defined, but it is just an theoretical exercise, and you don't really want to get into every variables like ambient air temperature, air flow, material thickness, barometric pressure, etc.. Besides, the latent heat of water is more than 540 cal/gram as stated above if the water temperature is less than 212F, so there is a lot of little detail that I did not get into.

Anyway...
For the second question, this is what I expected.
200W*h = 171 kcal, and since we know each gallon of evaporative water is ~2000 kcal, so the answer is simply 170/2000 gal = 10 fl. oz (less than the volumn of a soda can).

For the first question, let's assume the tank volumn is 40 gal, then the pull down is:
171 kcal @ 2kcal/(gal*F) <see above>
so answer is ~2 F.

 

[tangir1]

Oh, forget to mention that hdtran got the point of my thread right on. If you can get evaporative cooling to work, it can be a very good advantage. Imagine that 10 oz of water is equiv. to running a 200Watt (~1/4 HP) chiller for a full hour?

But the down side of evaporative cooling is that you really need to make sure the top off water is very pure, otherwise the junk going to accomulate to pretty high level if you don't do water change regularly.

 

[ryan72]

Tc=(5/9)(Tf-32) or Tf=(9/5*Tc)+32

also one gallon of FRESH water weighs 8.35 pounds
sea water weighs more

and one calorie is enough energy to raise one gram (mL) of FRESH water 1 degree celsius

finally the latent heat of FRESH water is different from that of sea water

not to be a science geek, but although the differences will be slight, they will matter greatly especially when dealing with large volume tanks and could throw your results off.

 

[tangir1]

Good points.

>... and one calorie is enough energy to raise one gram (mL) of FRESH water 1 degree celsius

Actually, if I remember correctly, it is from 14.5C to 15.5C, since the heat capacity of water is also temperature/pressure dependence.

However, I want to say that the above is just a simple illustration to give ball park figures, not a vigorous academic exercise. Besides, in my line of work, anything without one order of magnitude is good enough

(obviously, I don't build bridge)

 

[Acrylics]

...what? no brain candy??

James