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massgrad
15-04-2004, 09:53 AM
I will try to make this short and to the point.

Background: I am a graduate student in geotechnical engineering, my research involves freezing sand in the ground for undisturbed sampling. I used a NESLAB circulator to freeze sand in an insulated steel drum, but it is only 375 watts and not sufficient for large scale freezing.

The system: We would like a system that we could modify in such a way that we could use the evaporator as the heat sink in a closed loop system. The closed loop would consist of a pump circulating coolant through a freeze pipe and back to the refrigeration unit. We need a unit that is capable of temperatures well below freezing, the colder the better. We have access to government surplus where we could get a variety of refrigeration units but we don't really know where to start. Some key questions I have are:

What types of refrigerant provide the coldest tempratures?

If we get a system can we make it produce colder temperatures than the settings on the unit?

What are the problems associated with using a different refrigerant in a unit than what it was supplied with?

Any help would be greatly appreciated,

Chris

Peter_1
15-04-2004, 10:56 AM
What types of refrigerant provide the coldest tempratures?

Nitrogen is used here in Antwerp for a similar job when they rebuild the subway. You can rent a very small N2 vessel - at least here in Belgium - N2 is relative cheap and the setup is real simple and the freezing goes real fast.

If we get a system can we make it produce colder temperatures than the settings on the unit?

As long as you stays between the specs, no problem.

What are the problems associated with using a different refrigerant in a unit than what it was supplied with?

Don't do it or you will destroy the unit, especially if your not a refr. mechanic.
Most compressor are suited for only one refr gas and have to work between some prescribed margins.

Why not using a simple chiller, cool pure glycol or a glycol solution and circulate glycol in your heatsink?
We have an application where we pump pure glycol at -35°C through a system (Aluminium foil evaporating). You need a special pump for this but the chiller is a standard one.

massgrad
15-04-2004, 06:33 PM
Liquid nitrogen has been used by other people with great results. However, to get the desired quantity of frozen sand, they have used on the order of 60,000 liters over a 16 day period. We are looking for smaller quantities, roughly a third of what has been done in the past. So that puts us near 20,000 liters. Once a complete thermodynamic analysis of the soil and groundwater are done, a better estimate can be chosen. Also, you can't reuse the liquid nitrogen.

The specs - would I be incorrect to expect that the specs for a unit would be on the unit, say on the name plate?

I had a feeling the compressor would be a problem.

The simple chiller? We used ethylene glycol in our NESLAB unit. Where can I get information and specs on a simple chiller? What is a simple chiller?

chemi-cool
15-04-2004, 06:52 PM
hi massgrad,
a simple chiller is based on compressor, condenser air or water cooled,
evaporator, shell & tube, brazed or semi brazed and controls.

just type glycol chiller on your google bar and you will get over 100 answers.

chek them up and come back if you have difficulties making up your mind.

chemi

frank
15-04-2004, 08:34 PM
Excuse me for being thick but how do you freeze sand? :confused:

Brian_UK
15-04-2004, 10:48 PM
Excuse me for being thick but how do you freeze sand? :confused:
Nice one Frank, I'm glad someone else asked that one. :rolleyes: :D

Peter_1
16-04-2004, 07:02 AM
You freeze the water that is in the sand and/or surrounds it so that it sticks together as a massive block?
Also, when you start to cool, you will make a condensation point under the ground which will attract water. :confused:

massgrad
16-04-2004, 03:49 PM
To freeze sand: first the ground has to be saturated, there is pore space between the sand particles which is filled with water, since water expands 9% when frozen the soil has to be permeable enough to allow excess water to get pushed out by the freezing front. In sand, excess water is not attracted to the freezing front, but in clays, water is attracted to the freezing front which causes deformations therefore you can't get an undisturbed samples of clay by freezing (not easilly anyway).

I will check out the glycol chillers and check back this weekend.

Thanks for all your input,

Chris

shogun7
17-04-2004, 10:49 PM
Many freezing systems are made up of a brine cooling and distributation network plus a series of brine freeze pipes installed in the ground to be frozen, kinda like a septic system layout. The brine system network normally uses an ammonia compressor with ammonia to brine heat x-changer
This web site may be of some use for you!
http://turfgrass.hort.iastate.edu/pubs/turfrpt/2000/freezandthaw.html
Roger :cool:

massgrad
19-04-2004, 06:04 AM
Thanks for the info shogun.

Alright, so I've been looking at different types of systems. I guess my biggest problem is sizing. I'm thinking of taking apart a couple of standard refrigerators, since they typically use an ammonia refrigerant, to use as a prototype. How much cooling can I get out of a typical refigerator? I can't seem to find BTU ratings. Is there a place online or an equation I can use to estimate this?

Chris

Peter_1
19-04-2004, 06:22 AM
Thanks for the info shogun.

Alright, so I've been looking at different types of systems. I guess my biggest problem is sizing. I'm thinking of taking apart a couple of standard refrigerators, since they typically use an ammonia refrigerant, to use as a prototype. How much cooling can I get out of a typical refigerator? I can't seem to find BTU ratings. Is there a place online or an equation I can use to estimate this?

Chris

I edited it after re-reading your previous post.

I'm talking of perhaps 1000KW (I don't have a clue) cooling capacity and you're talking of +/- 100 watts.

Will never work the way you want to do it, especially because capacity sinks dramatically when working on low temperatures. Without going very technically, going from -5°C to -25°C --> capacity divided by 3.

Let's perhaps try to solve it in another way:
How big must your sample be and make a simple sketch, scan and post it here how you does it look like your sample under the ground. Cylinder, diameter, length, depth underground, how you push it in the ground, how much time do you have to freeze it (very important factor), ...

I have some literature about ground conductivity because I'm interested to use the ground as a huge heat exchanger.

Perhaps we can make here in this forum an estimate and perhaps someone from around you can make you then perhaps a cheap machine.
If you were closer, I could help you.

But still, I think the standard chiller , filled with glycol/water solution will be the best thing for you.

I changed a little bit my post after reading the following:
you know of a case where they used 60.000 l nitrogen in 16 days or 3750 l a day or 155l/hour. You need 1/3 of this or +/- 50 l. or 15 kg/hour
Boiling point a t 101.325 kPa is -198°C , That's very cold.

With this figures, there can be calculated precise - I can't - what the needed capacity should be by calculating how much heat is needed to vaporise 150 kg of nitrogen.
But what the diffrence will be if we inject a medium of -190°C versus -20°C can't be calculated. Lot of your cooling capacity will spread around again in the earth.

It's sampling direct in the ground, not digging some ground in your machine and freeze in then in the lab. :confused: :confused:
I think Marc can answer this with a fingersnap.

massgrad
19-04-2004, 07:16 AM
Peter,

I'll scan a sketch and I'll include some esimations on what kind of energy I will need. I have some good papers on thermal conductivity of soils if your interested. As far as a simple chiller, is that something I could make? We have a complete machine shop to make anything we need. If it's simple enough to buy a few components and build the machine ourselves, that is a definite possibility. I should have something in a day or two, thanks,

Chris

Peter_1
19-04-2004, 07:56 AM
Chris,

Thinks it's better now to wait till you post a sketch so that others can help.

But making yourselve will be almost impossible because refrigeration is a very specialised job. If you're not daily in this job, then it will be to difficult to do this.

Therefore, the use of gasses is legaly regulated. You need special tools and believe me, it's not that easy as you may think, it's something completely different as heating something.

But I don't understand something, if you have the funds to buy all the necessary items - and you will need some items Chris - you need to build some sort of a chiller/cooler, why then not buy a pre-made one? Will be much cheaper.


Peter

Peter.

massgrad
20-04-2004, 04:55 AM
Peter,

We have some funds, not necessarily unlimited. The deal is that we can get some used equipment from government surplus for free, we just got to pay shipping. If we need to spend a couple hundred dollars here or there for some parts that's no problem, but a few thousand dollars becomes a problem. So I've been searching for refrigeration equipment and havn't come up with much except refrigerators. I will conduct a search today for chillers and see where that gets me. I guess the biggest problem is that I don't know what to really look for as far as size goes.

Attached is a sketch of a typical single freeze pipe set up. The idea is that we would have a cased borehole to a depth just above our target zone. A freeze pipe would be pushed into the ground to a depth of about 2m from the bottom of the borehole. The inlet and outlet lines would be insulated from the bottom of the borehole to the chiller. Our freeze pipe is about 55mm in diameter and a disturbance zone is created that extends 2 to 5 times the diameter of the freeze pipe (depending on the properties of the soil), so worst case scenario is that the first 25cm of the frozen sand is useless to us. The diagram indicates a radius of about 1m for the frozen bulb, that is one option. Using that option, a core barrel would be used to get the samples we need. Another option is to freeze the sand such that the radius is at least 40cm and the frozen bulb would be pulled out of the ground and sampling conducted at the ground surface. Sampling depths will vary, but it will always be in saturated sand beneath the groundwater table.

Using Sanger and Sayles(1979) method of estimating frozen bulb growth, I got that the rate of energy extraction for 2m of freeze pipe at -35 degrees Celcius is roughly 340 W (seems low?). Including a loss of 30% due to sensible heat, and the energy extracted is about 2072MJ. I also got a time of about 8 days, which is definitly low. I used:

thermal conductivity of frozen soil = 2.8W/mK
thermal conductivity of unfrozen soil = 1.5W/mK
volumetric heat capacity of frozen soil = 2MJ/m^3K
volumetric heat capacity of unfrozen soil = 2.9MJ/m^3K
volumetric latent heat of fusion soil water = 138.3MJ/m^3

I had to estimate some parameters in the equations, and I think my estimates are a bit low, but that's what I got to go on for now. Let me know what you think, thanks

Chris

massgrad
20-04-2004, 11:01 AM
just a note, 2072MJ is about 1,962,000 BTU and that is the total energy required per the equations used to freeze the estimated bulb 1m in diameter. 350W is 1200 BTU/hr therefore it would take 68 days. Big dicrepancy in time!

massgrad
27-04-2004, 05:59 AM
Did I convert that right?

I need 350 Watts of cooling at -35C to freeze a bulb of sand in 68 days given that 2,000,000 BTU's are needed.

If I double that, does it cut my time in half?



Here is what I've found for equipment:

A Trane 80 ton chiller, 480V. Thats all I know about it, can't read the name plate.

A portable water chiller. Says it can bring down 120F water to 60F at 40gph. Requires 3.75 pounds of R12. Run by a 10Hp 6.6kW continuous rating diesel generator.

A Technical Systems Inc chiller. R22, 150lbs, then all I got is the load amp ratings for the compressor. Its a Copeland Discus compressor, 220V, RLA 107, LRA 565.

Is it possible to get these units down to very cold temperatures, like -35C or lower? How much cooling energy can I get out of these at low temperatures.
Am I wasting my time?

PobodysNerfect
28-04-2004, 04:34 AM
Chris,

Your freeze pipe will be able to extract more heat in the beginning.
Say, after 2 cm is frozen it will still be able to extract up to 4000 W, after 5 cm, it is down to 2000 W and when 1 m is frozen no more than 425 W (and could even be as low as your estimation - 340W).

This is of cause not exact calculations, but more a guide to sizing the equipment. If the average is about 500 W, I cannot see why you are thinking of using a 80 t chiller.

You should look for a small unit, which would give aprox. 2500W at -40 to -45 °C and use a heat exchanger to cool a brine solution.

Anyway, it will take long time. It the total energy extracted is 2072 MW (which - to me - seems like a good estimate) and the average extraction rate is 500W (J/s), then it will be 48 days, but as it is possible that the heat extraction rate actually is lower, you need some other measurements to verify if all is frozen.

You cannot lower the time by installing more capacity, but only by lower the temperature. You must be sure to have a good circulation rate of the cooling medium, e.g., brine, to ensure a good heat transfer rate in the freeze tube.

Saludos

Jan

chemi-cool
28-04-2004, 02:07 PM
hi jan,

one correction that makes a different - if the diameter is one meter, and assuming cooling is from all sides then you only freeze half a meter.

saves time and caoacity.

how about eutectic plates, constant cooling capacity.

chemi :)

massgrad
29-04-2004, 03:59 AM
Thanks Jan.

The sizing is my biggest problem, I don't know what an 80 ton chiller puts out. Judging from the size of it, it looks too big, but it's free! I've got some more searching to do, thanks for everybody's input. I guess I'm looking for a brine or glycol chiller that puts out ~500 Watts of cooling at very low temperatures. Is there a ton rating that I should shoot for or is it really all dependant on the particular machine?

Chris