New Build, 3-Stage Cascade. Need some pointers.

[Russell_hq]

OK guys, I may be building a 3-stage cascade in the near future and need some help/advice from you guys that have allready built a cascade (doesnt have to be 3 stage). I've run some models to try and simulate for a 200Watt load at -128ºC (b.p of R14) to give me a strating point.

I wanted to try and use R50/R1150 mix depending on what happens with the R14 (cost/availability) but I cant simulate this because my software doesnt have the data for R1150 :(

First Stage Description
For first stage I want to use R507a with LP @ 1.01 bara and HP @ 16.5 bara
I would be using a Hitachi horizontal scroll compressor (ZS series) 1.5 or maybe 2HP (not much price difference them).
The condenser will be rated around 1.5kW and the evap will have to handle between 600-800 Watts. I want to use a CPEV for the expansion device, as these appear simpler in operation compared to TXV's.
I will also stick on a suction line heat exchanger to try and keep evap temps down.
I will include a HP cutout switch that will cut power to all three compressors if there is a fault (e.g fan stops)

Second Stage
The gas for this will be R503 (or R23/ethane/R1150 mix depending again on price/avaiability). LP @ 1.01 bara and HP @ 9bara. I will add a desuperheater to the compressor outlet to bring gas temps down to around 25ºC before going into an oil separator then to the condenser (this will reduce load on the 1st stage evap).
I would want to use a CPEV again on this stage but am unsure about working temperatures so may have to opt for cap tube (cap tube will be a trial and error affair :( ).
I will add a suction line heat exchanger to this stage for same reasons as before. The evaporator will be handling between 300-400Watts
I will add the same pressure cut out measures, this time cutting power to the second and third stage compressors only. I may alos add an expansion vessel (would a large accumulator work for this? eg 4L)

Third Stage
The gas for this will be R14 (or R50/R1150 mix, same reasons). I will use the same desuperheater idea as before aswell as oil separator. For the expansion device, I want to use CPEV but as before may have to be cap tube (this is where I need your advice). I will add a suction line heat exchanger for same reasons and an expansion vessel. The HP pressure cutout will switch off only the 3rd stage compressor.


I will add extras such as HP and LP gauges on all three stages and things like driers. Your advice on parts and your thoughs from experience will be much appreciated. e.g. things like heat exchanger sizes etc..

I look forward to reading your replies :)

[Tonic]

Firstly, my knowledge isn't at high level, so, I may be wrong

1). Why do you want place desuperheaters before oil separators? Don't you think that a warm oil will go
to down in oil separator more easily?

2). I suggest you to try build only two stages, high stage should be a autocascade (R507 on first stage
and R1150/R170/R23/R503 on second stage) that should make ~ -80*C. According to this P/T chart of R14 :

http://www.airliquide.com/en/business/products/gases/gasdata/images/VaporPressureGraph/Tetrafluoromethane_Vapor_Pressure.GIF

With -75*C and 15 bar you can condense R14. R14 evaporates at -128*C at 1bar.

A two stage cascade with -128*C at idle..

[Unknown_road]

How do you know for sure that r50 and r1150 will mix? I doubt it...

I would do first stage with a TXV, it's very easy that it can cope with changing loads and you don't have to tune it manualy.

A cpev will work at those temps. the size of accumulator depens on your systems design and the static pressure that you find acceptable. 4L is probably enough though.

[Russell_hq]

The desuperheaters take the hot gas out the compressor (80ºC) down to around ambient temps, 25ºC before it goes to the condenser to condense at -35ºC or -80ºC. This will lessen the load on the inter-stage heat exchangers (which is a good thing). I would rather have the oil cool when it gets flushed back to the compressor, rather than having it sit in the oil sep with 80ºC gas flowing above it. Thats only my opinion, I would rather have someone more experienced have a comment on this. Im not sure about a hot oil separating more easily, a colder oil is more dense and this may make it separate more easilly.

Autocascade is a nice idea but I dont want to go down that road, keeping it simple as I can so less can go wrong.

[Russell_hq]

Yeah they will mix. I have mixing parameters for methane/ethane so I dont see why it wouldn't work with ethylene, they are from similar families of gases so I would say it will work. There is little other choice :(

[Unknown_road]

Yeah they will mix. I have mixing parameters for methane/ethane so I dont see why it wouldn't work with ethylene, they are from similar families of gases so I would say it will work. There is little other choice :(

the fact that ethane and methane mix doesn't say anything about ethylene and methane mixing. ethylene has a whole other chemical structure then ethane or methane. double binding, other angle of Hydrogen atoms etc.

desuperheater placed before oil sep is good.

[Russell_hq]

Put it this way, they gotta mix or its going to make things a whole lot harder ;)

I know what your saying about the gases, but I would doubt it very much if they didn't mix. Its not like ethylene is a million miles away from ethane. I also have mixing parameters for methane, ethane, propane, butane and propylene, I believe that the hydrocarbon gases are just good at mixing with each other.

[Tonic]

I would rather have the oil cool when it gets flushed back to the compressor, rather than having it sit in the oil sep with 80ºC gas flowing above it. Thats only my opinion, I would rather have someone more experienced have a comment on this. Im not sure about a hot oil separating more easily, a colder oil is more dense and this may make it separate more easilly.

80*C oil should return in main suction pipe, there it should be cooled.. but
it adds a load for low pressure gas, but it also improve desuperheating
discharged gas that doesn't contain hot oil (ok, it contains, but a small
amount, still DIY oil separators aren't efficienty..). But if you say that
placing oil separator before desuperheater is a better option, then ok



Autocascade is a nice idea but I dont want to go down that road, keeping it simple as I can so less can go wrong.

I'm thinking that to get a fine tuned three stage is a pain in the ass.. in my
opinion, tuning two stage cascade with autocascade on high stage is more
easier than tuning three stage with normal phase change units. Remember,
it's my opinion and I believe that you want a intelectual discussion ;)

[bh2k]

1st stage autocascade hmmm sounds interesting, but sounds like more pressure then I care to deal with.