Cooling Massive Heat Loads

[danwill51]

Now that I have taken the time to get some education on how to use the Forums I will ask this question again and explain a little better.

I need to heat and cool about 3000 lbs of glass panels (solar) cycling them between 85'C and -40.

The cycle time with dwells is between min 3 hours and max of 6 hours.

I have looked up the specific heat of glass and it is .20 so I have a factor of 5lbs per degree F per BTU. Right?

If I wanted to do 180'F per hour ramp times I would need around 110,000 BTU-Hr's of cooling (excluding the box losses etc) I know I also will need a ton of airflow as well.

Do these numbers seem right?

Thanks in advance for any help
DW

[frank]

Not one of my speciality areas, but, I would consider that you could use ambient forced cooling to start with and then refrigeration to finish with.

High glass temps of +85C would have a dramatic effect on any refrigeration system.

Would the glass suffer from thermal shock if cooled too quickly?

[Peter_1]

Cooling down 1360 kg glass with a DT of 120 K over 3 hours gives me roughly 47 kW needed cooling power

Placing SI units between brackets is easier to read for some ;)

You also mixed between the units: is it it from 80°F to -40°F or is it in °C ? A big difference

Your compressor will be too small if you take your BTU's.
Don't forget you will have to choose a compressor given the needed power at an evaporating temperature some degrees lower than your lowest temperature.

[Gary]

I would think the first question should be, how quickly can glass be cooled without shattering it?

[Peter_1]

I think you need this for strength tests like they test PCB's going through several cooling heating cycles?
Or is it part of a real production method?

[Peter_1]

Be aware that you will need somehow a protection if the evaporator is heated to 80°C when there's still refrigerant in it.:eek:

[lowcool]

aahhh pump down,but id first bring it down with evaporative and then chuck it in -40 bath to see what size grit we can have for expensive sandpaper,but wouldnt only a few panels be required for this,that tonnage is pretty extreme for solar panels unless you have a new product to market for nasa

[lowcool]

did pull downs for codan radios,they tested every unit that was made from ambient to 75 deg.c. down to -34 deg.c. i think they only pushed 6 sets through at a time,small chamber and lots of grunt,second test chamber they had from memory but that was years ago.i reckon the tests where at least 6 hours as cycles were repeated.

[Peter_1]

Years ago, we serviced environmental chambers for Siemens military equipment.
Cycle time was 6 hours (I guess from +60°C to -40°C)

[danwill51]

They use 2 cascaded 30 HP compressors for the low end cooling but can switch to one compressor at the high temp cooling through a separate condenser coil. They use compressor-cooling loops with TEV or cap tubes fed to the suction line to keep the compressors from over heating when cooling from high temperatures. They have hot gas bypass valves to unload the compressors when the cyclic demand is too short to cycle the compressor/s.

You don't have a schematic of this, seems very interesting to know how they do this

[US Iceman]

Test chamber loads are not just the product being tested. You also have the mass of the floor, chamber itself, and anything else placed in the chamber, plus the air itself.

This sounds like a durability test, so the concern of the glass breaking (or not) is the manufacturers reason for the test, not the refrigeration engineers responsibility.

It would help if you could provide the test protocol. If the product is pulled down to the desired temperature and then soaked at that temperature for the dwell time that's one thing. If what you consider the dwell time is the time allowed to bring it down to temperature, that's something else.

[michaelm]

B”H

To reduce the size of the refrigeration system maybe thermal storage could be a solution. System is going to be used only for a few hrs. so is allowing for maybe 20hrs of the slurry ice storage mode. Slurry Ice has a very high energy density allowing to have a small storage. There are numbers of brines that have a freezing point below –40. Big refrigeration load for short time period is a good case for thermal storage.

[danwill51]

The test cycles from +85'c to -40"C once the product reaches +-2'C of the extremes it has to dwell for at least 10 minutes then it ramps back to the other extreme. It does this 250 times. I put this in my last post but it seems to have vanished. No more than a three hours before cooling is neede again not much time for storage.

Peter_1 looks like you over wrote my last post. Did I do something wrong?

Anyway I don't have anything electronic as far as schematics go. They use solenoid valves to switch between the cascade condenser/evaporator, and a standard fin tube evaporator in the chamber. The chamber evaporator also has a check valve in the return line to keep it from sucking the medium temp refrigerant back into the coil at really low chamber temperatures. Normally the single stage mode is used for humidity testing and the evaporator has a suction regulator to keep it above freezing. In humidity chambers they usually also have a small dehumidify coil without fins to remove humidity without cooling much.

I have to run I will paste my last post below luclily I write in a word processor becaues I'm an old dyslexic guy that can't spell at all...

earlier post below
Thanks for all the replies. The test is part of UL1703 for solar panels testing. Normally people only qualify one or two panels at a time. My client wants to do large numbers to prove their "new technology" works. There are three of the UL test I am trying to do. One is a 250 cycle test from +85C to -40C with a min cycle time of 3 hours and a max cycle time of 6 hours. The units only have to stabilize their temperature for 10 minutes at the extremes. Getting the cycles as close to minimum as possible is the difference between a one month verses a two-month long test. Because they want to run this amount every month it makes sense to make a chamber that will hit the minimum cycle time. I did find a chamber manufacture that makes units for this type of testing.

They make a chamber that holds about half the number I need and only charge $140K USD. They use 2 cascaded 30 HP compressors for the low end cooling but can switch to one compressor at the high temp cooling through a separate condenser coil. They use compressor-cooling loops with TEV or cap tubes fed to the suction line to keep the compressors from over heating when cooling from high temperatures. They have hot gas bypass valves to unload the compressors when the cyclic demand is too short to cycle the compressor/s.

It is all standard equipment for environmental chambers. Most environmental chambers are designed to remove heat from active loads at high ambient temperatures. Cascaded chambers can cool safely and regulate ambient temperatures above 100'C and still reach -70'c on the low end. I am no expert on cascaded systems. My boss seems to think I can do anything and who am I to argue. So I may be looking for a lot of help some day if he wants me to design a very large system. Everything I know about these things I have learned over the last 30 years just trying to keep them running, designing one is a little different...

[Tesla]

Hi DW
For efficiency you could have two chaimbers one hot and one cold, rather than heating and cooling a single chaimber. I think Michael has the right idea using glycol or brine as secondry refrigerant considering this will be more of a batch than continuous process. As for the heating solar or waste heat from refrigeration side could be used.

[Gary]

It seems to me that the heat from one batch could be transferred to the next batch and so on. Maybe have several batches going at once.

[knighty]

what about splitting them into 2 identical chambers, have an evaporator and a condensor in each one...

while one is heating the other is cooling
(bit like a heat pump)
then you witch it over and reverse it ?

guess it might need a condensor on the outside to help right down to the -40, and maybe some heaters inside to help with the last bit of heating ?


sounds like the best system to me.... you'll get loads of free heat anyway...

[US Iceman]

The test cycles from +85'c to -40"C once the product reaches +-2'C of the extremes it has to dwell for at least 10 minutes then it ramps back to the other extreme.


If that is the case, then I might suggest two separate chambers: 1 heated at the required temperature and 1 cooled at the required temperature. Then just cycle the product between the two chambers through a portal between the chambers.

Trying to do fast changes like this with a refrigeration system might be asking too much. not saying it can't be done, it might just not be practical.

[desA]

If Mohammed won't go to the mountain, then the mountain must come to Mohammed.

Two rooms - one hot, one cold - simple shuttle between... :)

[Gary]

And maybe an intermediate chamber where the two batches exchange heat, resulting in both being at intermediate temperature.

That makes four batches going at once.

[danwill51]

The shuttle idea is one I have used before I actually have a small shuttle style thermal shock chamber. The problem is that could exceed the ramp rate specification of 100'C/hour max but that much heat load would offset the cold chamber so quickly I would still need about the same amount of cooling to recover. I would only be gaining the inside walls and air volume in the cold chamber and compared to the 3000 lbs of hot glass not much gain at all. Then there is the shuttling of all that glass every hour and a half. bulding a contraption to do the moving might be interesting but would add costs. Using two chambers and switching the condensed and evaporator between them could be a money saver but I would have condensers running at over 85'C (185"f) I have a feeling the head pressures my be a wee bit high on any coolant I could get to -40"c on the cold side. I like the brine idea not so much for this project but I have an LN2 system that boil's off a lot of LN2 trying to keep the lines full of liquid. The gas at around -300'C could remove a lot of heat from a well-insulated slurry tank. Something to think about.

I have thought about using LN2 for this test it makes building the system super cheap but the operating costs are through the roof. I do use it in most of my chambers for quick cold transitions (5-10"C per minute) and it is cost effective when coupled with mechanical as long as the cold dwells are long but for this test I would run through over 100 gallons per cycle and that would hurt pretty bad when the LN2 bill came in.

Thanks for all the responses I am learning from the group and that is pretty good for an old dog like me>

[Gary]

At the very least, I would want to use the batch leaving the cold chamber to help cool the batch entering the cold chamber.

We can think of this as four chambers:

1. Cold
2. Warming from cold
3. Hot
4. Cooling from hot

The cascade evaporator would be in #1.

As a batch leaves #1 to #2, I would want the condenser to be in #2.

#3 would have its own heat source.

#4 would be cooled by outdoor air.

The outdoor air would enter #2, move through the batch of cold glass, then the condenser. From there the air would flow to #4, through the batch and exit outdoors.

[US Iceman]

Thermal shock testing is a tough duty for refrigeration systems. Cycling back and forth through extreme temperature ranges quickly is not something I would recommend. It would be much easier to independently control two separate chambers at the desired temperatures.

How big is the rack you are placing the panels on?

Mount the panels an a rack (that slides between the chambers on a track in the floor, or along a guide rail on the floor).

a) test the panels in one chamber, when test is complete, push the track (or automate it with DC motors) and allow it to move into an intermediate chamber (used to isolate the two temperature test cells).

b) close the doors behind you. Then open the doors to the opposite test cell. Close the second set of doors after the rack is in the opposite room.

c) now start the other temperature test cycle.

The intermediate chamber is used to isolate the two test cells. If you got ambitious you can do two sets at once. One rack in the cold room, and one in the hot room. The racks pass each other when they cycle from room to room. And it's automated with fairly basic control schemes.

Each time a new rack is placed in the desired room the pull-down or heat-up time could be programmed so the rate is not exceeded.

[US Iceman]

And... you would not need a cascade system. A simple refrigeration system could be used, but it would have to be designed to work within your ranges.

[danwill51]

I like the idea Not sure I could make it work because I also need to bias the panels to their rated current during the parts of the cycle that are in the opperating temperature range. There is also the problem with humidity and freezing up of the cold side chamber, a single chamber the cycling to hot and cold does a good job of defrosting the coils moving large racks in and out of a -40'C chamber be an issue with frosting. I do appreciate all the inputs I am thinking how to controll all this at once. I have a lot of experience with Watlow F4 controllers so I should be able to use the digital outputs to activate switching chambers, opening/closing doors, controling power and temperature control of both chambers. I have a while before I need to come up with final plan. I may end up having to install this a the clients site and spend their money on a $130K USD chamber.

[danwill51]

US Iceman,

My experience with running single stage chambers down to -40 hasn't been too good. I can get them down there but they have almost no capacity below -20C or so. I have a 2HP system that can only handle about a 200w load below -35C. I think with the high heat load I might need to go cascade. Since I am not a cascade designer (or anything like that) I will probably have to purchase one. Cascade Tech sells them but they are not cheap dose anyone know a company that sells premade cascade refrigeration units?

[US Iceman]

This is not a typical application. It is not the same as a walk-freezer. Heat load has nothing to do with needing a cascade system.

The use of cascade system might be required when the temperatures needed are lower than you can reasonably attain with either single stage or two stage systems. At -40°F (or -40°C) the system has to be designed to provide the required capacity at that temperature. You certainly do not need a cascade system for this temperature IMO.

If the system you mentioned does not reach the desired temperature the system is either not designed properly or is operating outside of the design point where it was designed to operate.

Most of the issues you mention are ones normally encountered in this type of work. They are all solvable, if approached right.;)

[Peter_1]

Peter_1 looks like you over wrote my last post. Did I do something wrong?
I am really sorry , you haven't done anything wrong.
I realize now it's me who pushed on the Edit instead of the Quote button.
Oh, this is so stupid of me.
Sorry my friend, I will better look out the next time.

[Peter_1]

I don't see why a room on -40°C wouldn't work with a 1 stage on R507? You gain +/-1K compared with R404a when reaching atmospheric pressure.
You can work with a DT of 7K over your evaporator.
Regulate atmospheric pressure with a capacity control or a VFD

[danwill51]

Peter_1 No problem I kinda thought that is what happened. Just be careful I hear they will reduce your moderator pay if it happens too often:) Seriously, I appreciate the time you spend to be a moderator. It is people like you that keep the forums up for everyone.

I may have used the wrong wording as to the single stage issue. It is not the capacity of the refrigeration system but more the ability to remove heat quickly when there is not a big difference between the evaporator minimum temperature and the product you are trying to cool. Of course the bigger the difference inside verses out side temperature the more losses so capacity always goes down some on the low and high ends.

[US Iceman]

It is not the capacity of the refrigeration system but more the ability to remove heat quickly when there is not a big difference between the evaporator minimum temperature and the product you are trying to cool. Of course the bigger the difference inside verses out side temperature the more losses so capacity always goes down some on the low and high ends.


Huh? You lost me.

[RefrigNoob]

Why not have a diesel burner do the hot room, and a standard cooling unit do the cold room?
A small 2 stage package burner will easily do 85 deg C with good accuracy.

If you want all this to happen in a single room (though 2 rooms make more sense) you can quickly evacuate the hot air with fans, get the room to ambient and then start refrigirating the room.

[danwill51]

US Iceman

I guess it all comes down to design limitations on systems that have to operate at a very wide temperature range using one or two evaporators and same refrigerant charge. I have heard there some newer systems with servo controlled TEV's that work much better at the low end but I have never seen one working. I looked at the spec sheets on several environmental chamber types and the heat load capacity always drops dramatically at the low end of their range around -40. It is more than just the extra loss because of the inside verses out side chamber temperature difference.

Refrignoob
I hadn't thought of using an oil burner for heat. There should be plenty of energy there I am just a little wary of the controlling of such devices. I need a +- 1"c control and smooth temperature transitions that will be difficult using any heat source I can't toggle off and on rapidly. I also won't be able to use the multi chamber design because this is just one of three different test the chamber has to do. One also has a bias placed on the panels so it has an active heat load and a very high humidity requirement as well. The last test is called humidity freeze test that requires a soak at 85'C 85%RH then cycles down to -40'c at a rate of more than 1'c per minute and cannot use external air because they want the humidity to freeze in/on the panels. These tests are not what we call in the test world thermal shock. Thermal shock tests require 50'c per minute or more air temperature change.

Thanks to all for the suggestions I am having to put this on the back burner as prioritys have changed again...