Using Condesing unit and High load and temperature?

[evcelica]

Hello everyone,

I am a mechanical engineer, so I am a little new to some of this commercial refrigeration stuff but have a problem even some experts can't figure out. But I'm trying to see if my theories are correct. I am using R404a units to cool liquid CO2 for a particle detector cooling system, and controlling them with a PLC.

My problem and what I think will fix it:
I have been asked to design a cooling system which could keep the CO2 (my load not my refrigerant) at temperatures from -40F to 60F. My 3 chilling units are designed to only work at temperatures below 0 F. If I need to hold my tank temperature at something above 0F I guess these units won't like the high temperature/pressure in the return suction line, and may cycle on and off? My thought is at high load temperatures: I have a CRO valve to keep the pressure down, as well as my TEV has an equilibrium line that will bypass the evaporator and mix cold liquid/gas in with the much too hot gas coming out of the evaporator. My question is, is this equilibrium line able to flow enough cold refrigerant to mix with the 'too hot' superheated gas from my condensor to drop the suction line temperature down to the maximum reccommended by the manufacturer or are they just too undersized? Will I need a discharge bypass valve or some other custom controlled solenoid valve system to keep the suction temperature down or will this TEV with the equilibrium line work?

Keep in mind I do not care if it is horribly inefficient whicle doing this, I just need it to work and hold temperature. I also have a liquid seperator on the suction line.
Any other suggestions would be appreciated. Thanks a lot in advance for sharing your knowledge with a refrigeration newbie.

Erik

[monkey spanners]

The equalising line does not flow any refrigerant, it is just so that the txv can sense to evaporator outlet pressure, they are used when the evaporator is restrictive, in place of an internally equalised valve (which would be used on a non restrictive evaporator)

What you need is a crank case pressure regulating valve, same as would be fitted on many freezer applications to allow the room to be pulled down from ambient temperatures.

[evcelica]

Thanks for the reply, I see the equilibrium lines dont flow, just a small orifice for equilization from high to low sides. I appreciate your knowledge, Thanks!!

The Crankcase Pressure regulating valve is what I called the CRO. I have those and they will help with the pressure but I still need to reduce the temperature as well. I believe I need to use a desuperheating TEV. and all should work, but I ned to size this and set it at a higher superheat than my TEV which feeds the evaporator. Any other thoughts ar suggestions on this because I've never seen this stuff before and don't really know whats normal in these type of extreme superheat situations where you are using equipment in a manner which is wasn't designed for. I may need quite a large desuperheating TEV?

[evcelica]

Will the Crankcase pressure regulating valve work by itself even with high temperatures? or is this only for short periods of time? Since its made to pull down not to hold?

[monkey spanners]

Just to be clear, even though the equalising line is called an equalising line it does not equalise any system pressure from high to low side.
For the txv to work propperly it needs to know the temperature of the refrigerant exiting the evaporator, hence the phial/bulb and it needs to know the pressure hence the equalising line, this line goes to one side of the bellows in the txv and the gas in the phail exerts pressure on the other side of the bellows, the bellows then push a pin to regulate flow of refrigerant into the evaporator.

I see know reason why the crank case pressure regulator wouldn't work by itself, i know of one make of equipment that is designed to be a freezer and a chiller depending on what temp it is set to and uses cpr valve to protect the comp during high temp opperation. But it has been designed and not just assembled, so in your system some trial and error may be needed.

The cpr valve will stop the compressor being overloaded by too high an suction pressure, if though the system runs with too high a compresor discharge temperature when running in this mode then a small liquid injection system would be something to look into.

[Brian_UK]

Sporlan do a compressor cooling valve, the Y1037, which may be the answer that you are looking for.

Download document here... http://www.sporlanonline.com/10-207.htm

Also, click on literature on the left hand panel of the website for a good range of information.

Yes, the CRO responds to pressure only not temperature.

[mad fridgie]

CPR, always a good idea,
Why do you not just install a 3 port valve in your CO2 line, (mix hot CO2 with cold CO2) to give required supply CO2 temp.
Bit hard to help when duty (size in KW) is not known! is this a baby or a monster of a system.

[evcelica]

Excellent, thanks.

By liquid injection, I'm thinking of using another expansion valve, sized a pretty small, I'll have to calculate this. I would then split the high pressure side to two TEVs and set at 10F superheat, and one set at 20F superheat. The 10 superheat would feed though the evaporator, and under high loads would go full open yet still be too much load and superheat would rise to 20F or higher. This is where the second TEV would bypass the evaporator and directly inject into the suction line, and its bulb would be after its dumping point, this would have to be an internally regulated valve to work and hold the much lower temperature. This seems like it should work to keep the temps down. The Crankcase pressure regulator should keep the pressures down. Will this allow me to operate a unit at evaporator temperatures 60F higher than the unit is supposed to cool?

Thanks for all the input guys!

[evcelica]

The system is around 20kW. The CO2 is what I'm cooling with the 404a. Mixing Hot CO2 and Cold CO2 is a good idea, but I would need more advanced controls, another pressure vessel (BIG $$$). Also it needs to be saturated liquid so I think mixing them with a few hundres psi difference would not work as my Hot would shoot into my cold tank. Also If I can get away with a throttling valve, id rather spend the few bucks.

Great thinking though, thinking outside the box, I like that.

[mad fridgie]

Are you using the CO2 as primary refrigerant (vaporizing) or a secondary refrigerant ( fluid only)
How are you moving the CO2 around or is the process flooded by CO2 and you are thermosyhoning via a R04a heat exchanger

[cool runings]

Excellent, thanks.

By liquid injection, I'm thinking of using another expansion valve, sized a pretty small, I'll have to calculate this. I would then split the high pressure side to two TEVs and set at 10F superheat, and one set at 20F superheat. The 10 superheat would feed though the evaporator, and under high loads would go full open yet still be too much load and superheat would rise to 20F or higher. This is where the second TEV would bypass the evaporator and directly inject into the suction line, and its bulb would be after its dumping point, this would have to be an internally regulated valve to work and hold the much lower temperature. This seems like it should work to keep the temps down. The Crankcase pressure regulator should keep the pressures down. Will this allow me to operate a unit at evaporator temperatures 60F higher than the unit is supposed to cool?

Thanks for all the input guys!

You are redesigning somthing that is done already.

You are also trying to over complicate it by doing
it your way.

Keep it simple.

coolrunnings

.

[lawrence1]

You need to keep in mind the pressure of co2 at 60f,,,750psi.Most commercial supermarket systems here release at 650psi.

[evcelica]

You are redesigning somthing that is done already.

You are also trying to over complicate it by doing
it your way.

Keep it simple.

coolrunnings

.

Coolrunnings,

That is why I'm here, if there is already something on the market which does this that would be great, That is what I'm trying to figure out.
Problem is I have no experience in the refrigeration industry so I have no clue what products are out there, and the people I've spoken with, even the manufacturers of the units don't seem to know enough to give me good information on what will work, which is why I'm here, trying to find out.

The CO2 is the process side, not my refrigerant. This will be in a closed system designed to 1200psi. The R404 (in commercial units) is what will be cooling the CO2 through custom heat exchangers in a pressure vessel.
Cooling down the 404 from 60F to say 10F after it comes out of the hot heat exchanger it my goal. So I'm guessing I need either a desuperheating TEV or I can just use a solenoid valve and directly bypass the evaporator and control the liquid injection myself.

I will be monitoring the temperature and pressure of my liquid and suction line and could program my PLC to inject liquid accordingly to keep temps low enough for the condensing units not to overheat.

But like I said if there is an industry standard way to do this, that is what I'm looking for, please share coolrunnings or anyone else, as I know next to nothing about this stuff.

Thanks again for the replies guys, Have a good weekend.

Erik

[mad fridgie]

Do you have this machine?
The heat exchanger (fixed surface area), I presume some form of finned to tube inside a vessel.
You are saying your load is constant 20Kw but at possible different temperatures. (thus different evap pressures on the refrig unit)
your concern is that your refrig unit is going run out of spec.
So options available, vary surface area (increasing TD to reach load equalibrium) This done by splitting your heat exchanger, with each circuit having its own liquid refrigeration supply (sol valve and TXV) or you install additional medium temp compressor, switch between comps when desired load is required.
Install CPR, which will end up flooding your coil, so you need to know charge required for flooded coil, then is your reciever able to accept the increase charge.
basically you need to load of a drawing so we can understand the process and showing mins and max of temps and load profiles.

[evcelica]

I do have the machine.
Heat exchanger is tubes inside a vessel. 11.2 sq.ft. of surface area. CO2 condensing on shell side, r404 evaporating on tube side.

I can not vary the surface area, as the heat exchangers are already here. The capacity of the system is up to 20kW. Load would only be around 2-5kW. I have three condensing units (different sizes) which can be switched on/off as well as a 6000W trim heater to control the temperature based on load. Loads can vary, and I can compensate with the trim heater to hold a load constant if neccessary. Temperatures of process can vary from -40 to 60F, but units are only supposed to refrigerate at temps from -40 to 0F.

I'm guessing at the high process temps the load would be too much and have too high of temperatures/pressures on the suction line since it may come out near the process temperature?

Attached is one condensor and lines showing the maximum process temps and maximum suction line temperature (I'm assuming the same temperature that the units are designed to cool, I'm sure it can handle a little higher). The design pressure on the suction side is 125psi, I dont know what pressure they are supposed accept but I can adjust this with the CPR Valve. which should probably be placed somewhere near the evaporator (of course after all the liquid injection ect.) if it will cuse flooding in everything before it?

[mad fridgie]

Just to clarify, your 3 condensing units that you control, are on a single CO2 heat exchanger or 3 seperate Heat exchangers?
If on a single heat exchanger, just install a dedicated high temp condensing unit (SST -25C to +10C)

[Brian_UK]

At 60°F your R404a is at 126.8psi which I think is close enough to your design of 125psi.

Put some faith in the crankcase pressure regulator. Set it correctly and the units should run safely.

Trying to add liquid injection when you don't appear to really need it is just added complexity for the sake of it.

[evcelica]

The Units use three seperate heat exchangers.
I will run the units with the crankcase pressure regulators, If they keep tripping off, then I will add liquid injection. Sounds good but my only worrk is how much harder it will be to try to install the liquid injection after everything is running. I will need to reprogram my PLC, put more wiring in, ect. If I just instal it and only activate it if I need to this may be easier that having to go back and reinstall everything. Unless you are pretty sure I wont need it.