Compressor Cycling on SS system

[aussie_guy00000]

Hey Guys,
I've been working on a design for a kind of SS system and there's something that I was going to try and just thought i'd it run past some of the HVAC guys that frequent these forums to see if i'm on the right track. Most SS systems just have the compressor running constantly when switched on, meaning it's drawing pretty much max power whilst in use, unlike an air con or fridge that cycles on and off to match the load on the system.
The system i'm building uses a 0.75hp rotary (it's that big cause it is used for more then just the CPU) and if that were to run constantly it would use some serious power, and the objective of the system is to be convient and be able to be used 24/7 if required. So what i'd like to do is build some capacitance into the system in the form of a largish liquid accumulator that feeds either a TXV or cap tube, but the system would basically cycle the compressor on when the accumulator is almost empty and off when it's full of liquid. Obviously this would mean there would have to be an almost equivalent volume on the low side of the circuit, and the pressure differential would change as the liquid level changed.

Forgetting for a moment the controls that would be needed to detect the liquid level and such, is this feasible or am I just off my rocker?

Failing this idea, is there any other way that you can think of to maintain a relatively stable evap temp and not have the compressor running constantly?

Please be gentle :slap:

[DetroitAC]

Well, your a bit off your rocker on the approach your thinking of. I think cycling the compressor on and off though is a fine idea, but you're going to need a different form of thermal inertia (your term capacitance is also a good one). The problem with your idea is the ability of the low side of the system to remain at a low pressure. It is not feasible to do this:
Obviously this would mean there would have to be an almost equivalent volume on the low side of the circuit, and the pressure differential would change as the liquid level changed.
The reason is that the volume that refrigerant vapor occupies is about 140 times larger than the volume occupied by the liquid (these numbers are for R402A at -40C evap, 30C cond).

For a load of 200W at -40C, you need a mass flow rate of about 1.75 g/s of R402A. To keep the compressor off for even 10 minutes this would mean 1.05 kg refrigerant must flow, which will mean 0.95 liters of liquid, and 136 liters of suction gas if you could somehow keep it at a low pressure. A 136 liter accumulator isn't going to work, the pressure will rise as vapor is added, so the accumulator must be bigger than this.

It can be done for sure, but you'll have to do it another way.

[jinu117]

Errr...
Variable speed control compressor comes to mind.

[aussie_guy00000]

The reason is that the volume that refrigerant vapor occupies is about 140 times larger than the volume occupied by the liquid (these numbers are for R402A at -40C evap, 30C cond).


Thanks Detroit, that was my main concern, after I posted this it did dawn on me that there would have to be a significantly larger volume on the low side to even attemp to maintain the pressure differential.


Errr...
Variable speed control compressor comes to mind.

Unfortunately the aim of the design is to keep costs down, so it pretty much rules this out. Btw i've already found a supplier of very reasonably priced horizontal rotary compressors, can't wait to get my first shipment to try them out :toast: .

[jinu117]

Thanks Detroit, that was my main concern, after I posted this it did dawn on me that there would have to be a significantly larger volume on the low side to even attemp to maintain the pressure differential.


Unfortunately the aim of the design is to keep costs down, so it pretty much rules this out. Btw i've already found a supplier of very reasonably priced horizontal rotary compressors, can't wait to get my first shipment to try them out :toast: .

errr... did cost of refrigerant go down that much ? :)

[Xeon th MG Pony]

VFDs arent all that expensive

[_HL4E_HalfLife_]

Use the compressor to chill a large volume of liquid in a well insulated box.

[Jort]

just like _HL4E_HalfLife_ saids, chiller all the way.

no way you can switch a singlestage on and off without having problems with your temps. or like jinu117 saids, frequenty controlled compressor.(not worth the money for that application)

[aussie_guy00000]

Yeah, I thought that might be the case, but It was worth a try.

No, it didn't jinu117, that's why tha large volume accumulators are out as well, oh well, will have to stick with the try and tested way of building a SS.

Could you point me in the direct of a "not that expensive" VFD Xeon?, Not sure what you can get them for in the states, but pretty much everything here is a ***tload more expensive, especially compressors :(

Thank you for all your help and opinions, I know what I have to do now :toast: .

[C-BuZz]

Refrigerators do switch on & off however there is usually quite a lengthy period between switching on & off which allows the pressures to balance.

The system you are talking about would be switching on/off within 10/30second period as the temps increase fairly quickly when the compressor is switched off.

Switching a system on/off like this over a short period of time can cause the mains to blow & has done at my place quite a few times using large compressors, without allowing the pressures to equalize. Im guessing because the compressor draws to much current trying to start up with such high "high side" pressures than lowside, if you know what I mean. Not to mention some strange things happen when you switch the system on/off in such a short amount of time.

My 2c & im probably wrong hehe.

C-BuZz

[Nagaru]

Most compressors can not be cycled more than every five minutes and even at that rate it won't last long. I think a chiller makes sense, I am hoping to do something similar so that I can save on electricity.

[DetroitAC]

Well, I'm going to have stir up the pot and say that cycling chiller on and off won't save electricity if you are getting the same amount of cooling at the cpu.

The power consumption of a refrigeration system is proportional to the amount of temperature "lift" the system is achieving. This manifests itself in the compressor, and you could say the power consumption of a compressor is proportional to the pressure ratio it is working against.

Let's say that you want to keep your cpu at -40C. The chilled liquid will have to be colder than -40C, so let's say it is -45C because of the (thermal) resistance of the heat exchanger. Then the heat will have to be removed by a refrigerant evap that is running colder yet, let's say -50C. So, I'm saying that a direct die evap might get the cpu to be -40C by operating at -45C, but a chiller loop added in between (with the additional dT necessary at each heat exchange) will make the evap operate at -50C. These are just made up numbers for the argument.

If you want a refrigeration plant to run only part of the time, and remove the same amount of heat...it's going to have to be increased in size according to the duty cycle you want. If a single small compressor running constantly can hold -40C @200W capacity, and you would like the compressor to only run 1/2 the time, you have a 400W system (roughly) + inefficiencies from the pump and increased power to overcome the additional lift.

It will be less efficient, but quieter half the time? Maybe noisier the other half of the time??

[mtl_hed]

Detroit: I think the idea is that in our application load is not constant.

The intention would be that the compressor would not be running most of the time when load is very low. When benching/games are started the compressor would be running non-stop.

I believe the OP intent was to design a 200W system that only needs to be run a fraction of the time instead of a 24/7 200W system.

[epion2985]

Like DetroitAC said, the power consumption is depandant on the load in the end of everything. Look at any compressor spec sheet under power consumption, and it lists it as in reference to the temperature you are running. As your load varies so does the temperature your system can hold it at (with sertain limitations).

http://de.refrignet.danfoss.com/TechnicalInfo/Literature/Manuals/06/NF11FX_R134a_220V_50Hz_04-05_Cg43c422.pdf

http://img86.imageshack.us/img86/8703/7061ik9.jpg

[aussie_guy00000]

I'd have to say that Detroit is spot on unfortunately. Well said mate. I guess I was just in denial :D

[Nagaru]

I understand that the more heat I want to remove the bigger compressor I will need, and that a given compressor will draw more power. My goal was to design a system that could handle full load well, but also would not run as often when the system was lightly loaded, such as at idle where a loop with a powersaving processor and a graphics card, the load could easily be half of that of fully loaded playing a game. I will leave my system on 24/7 and think that at low load times power could be saved.

[jinu117]

I understand that the more heat I want to remove the bigger compressor I will need, and that a given compressor will draw more power. My goal was to design a system that could handle full load well, but also would not run as often when the system was lightly loaded, such as at idle where a loop with a powersaving processor and a graphics card, the load could easily be half of that of fully loaded playing a game. I will leave my system on 24/7 and think that at low load times power could be saved.

One thing you forget to think is that capillary tubing system, will change compressor load based on load on your evap (automatically... :P) My unit can range from 170w-240w depending on amount of load as is. Just something to think about :")
Most efficient way I can think of really is to have variable speed compressor that trys to work at it's most efficient range for each heatload... maybe than that 170w might go down to 100w...)

[DetroitAC]

I think a refrigeration plant that saves energy when the load decreases can be done, and is probably a good idea. We just have to think of the way that will get the benefits for the least cost/complexity/size/noise.

The variable freq drive is a good idea, but that requires a 3 phase compressor. I did a quick check and the smallest 3ph compressor in my catalog is a 1hp. The VFD is no problem, I have a nice Hitachi that I use to run my milling machine spindle motor, lovely piece of hardware and very easy to control using signals from controllers,PLCs, etc. It was about $200 on Ebay.

Maybe it'd be better to get some savings by using a hot gas bypass, and keep a small simple system?

[epion2985]

One thing you forget to think is that capillary tubing system, will change compressor load based on load on your evap (automatically... :P) My unit can range from 170w-240w depending on amount of load as is. Just something to think about :")
Most efficient way I can think of really is to have variable speed compressor that trys to work at it's most efficient range for each heatload... maybe than that 170w might go down to 100w...)

the hvac engineer I live with said back in the says when freq drives came out they all thought thats it, variable compressor speeds will be everywhere. Today aperently the undustry rather put 10 compressors and turn them on and off depending on the capacity they need. Or use those multi cylinder compressors and just hold te valves open on some when they dont need the extra capacity. He said the industry tried it but the compressors kept failing due to the oil retun issues at low compressor speeds and the industry is to cheap to integrate a small oil pump, and doesnt want to change anything if it doesnt have to. He said there are so many things that can be done better but arent just because of the status quo and its very hard to break it, lot of people try and fail and in the end the only way it changes is when there is a serious need or problem.

So be carefull with slowing down your compressor as at low speeds oil seems to not want to come back and then you loose your compressor. If you want you can drill in to the shell and install and oil pump, its gettes alittle complicating.

[Nagaru]

One thing you forget to think is that capillary tubing system, will change compressor load based on load on your evap (automatically... :P) My unit can range from 170w-240w depending on amount of load as is. Just something to think about :")
Most efficient way I can think of really is to have variable speed compressor that trys to work at it's most efficient range for each heatload... maybe than that 170w might go down to 100w...)

I thought that on a cap tube system the load was constant because the restriction was a consistent size?

[martinjon666]

I thought that on a cap tube system the load was constant because the restriction was a consistent size?


But if your compressor isn't pushing anything into the system, or not as much as before, then it doesn't matter. These are rated to run at certain pressures, and temps, if the temps are high the compressor pushes harder to try and make up for the high temps, as the temps get low, it doesn't have to work as hard. Like your car, if it is sitting idle it can run all day on a tank of gas, but if your driving, and using its power to go somewhere, it takes more gas depending on if your doing up a hill or going Fast.

[epion2985]

I thought that on a cap tube system the load was constant because the restriction was a consistent size?

The restriction is constant, but the load is not. You can have xft of 0.xxx cap line and load Y and then start increasing the load, the pressures will change and so will the amount of force it takes for the compressor to keep compressing the gas, which will change the power consumption, which will keep rising as the difficulty of compressing the gas rises, untill the motor in the compressor will hit the maximum work it can do at which point it will hit its maximum power consumption by design.

[Nagaru]

Ok I understand thanks.