R-23 cascade system

Hi,
After designing and successfully commissioing a -85 Deg. F system using R-404A refrigerant with two-stage compressor, I have to now design a cascade system for -130 Deg. F evaporating using R-23 in the low stage and R-404A in high stage. The condensing temperature of R-23 will be -58 Deg. F. But whenever the system will be switched off for a few days, the temperture of R-23 in the condenser will rise.After checking thermodynamic properties of R-23 find that the pressure at 76 Deg. F (critical point) is about 700 PSIA. What would be the pressure at ambient temperature of SAY 105 Deg. F? What should be the design and test pressure for the cascade condenser for R-23 which will be the evaporator for R-404A.
Can any experienced person help?

You will need a simple expansion tank to increase volume, so decrease stand-still pressure.
You must stay below the max allowable pressure of each component. This will probably the LP-side of the compressor.
Also be carefully that you don't start with a very high LP which can result in too high HP.
Pressure at 105°F will be that corresponding to the pressure that can be found in a pressure chart as far not all the refrigerant was already evaporated before that temperature and superheated.
Try also to design the HP side as small as possible in volume so that you need as less as possible refrigerant.
What will you use for expansion device: capillary or TEV? If you install TEV, then you will need a liquid receiver which can gives troubles.

Start up sequence must be, but you probably know this already: start R404a and as soon LP drops below a presetted LP, then you start the R23 compressor. Lock after testing all the screws on the LP and HP safety devices so that others don't re-adjust your values.

Monitor temperature of the evaporator at half the coil length wit a thermocouple.

You probably will need to change the oil in the compressor. Ask this at the manufacturer.

Solder as much as possible all connections instead of flaring because the charge is critical.

A good practice also is tho install a protection on the discharge line short after the compressor. As soon you lose some gas, temperature can rise very quick to unacceptable values.

What is the application? Meisner coil in a vacuum chamber? Testing chamber? Blood cooling? Keep in mind that cooling capacity of R23 at such a low temperatures is very small.

I once made 2 cascade systems (R502/R13) going to as low as -100°C only to cool 45 ft or 15 m of 5/8 copperline in a vacuum chamber with 2 x 3 HP Copeland.

Thanks Peter_1,
Appreciate the quick answer with good duggestions.

This will be fairly large system (3.5 T.R.) at -90 Deg. C evaporating. I will be using 'Bitzer' compressor for R-23 which will have condensing temp. of -60 Deg. C to keep the discharge temperature in control. For R-404-A another two stage 'Bitzer; compressor will be used. 'Bitzer' have suggested that I should not go below -90 Deg. C as there could be problems such as brittleness of metal and oil return. The end application is crystallization of some pharma product at -70 Deg. C in a reactor. To achieve this I would need to cool the secondary fluid to -80 Deg. C at least.The R-23 compressor will be of 10 H.P. and the high stage compressor (two-stage) will be of 40 H.P. for -70 Deg. C evaporating and 25 Deg. C condensing.TEVs will be used for both the systems.

The system will have a shell & tube DX chiller using synthetic hydrocarbon fluid (probably 'Paratherm CR') The condenser will be shell & tube with the R-404A evaporating inside the finned tubes of the condenser. I can probably oversize the condenser shell which will eliminate the need of an expansion tank.

My problem is that the pressure chart of R-23 doesn't give pressures beyond 76 Deg. F. Anyway as the condenser will be insulated with 150 mm thk. polyeurathane (for -60 Deg. C condensing) I do not expect the R-23 temperature to rise too quickly.However, in case of a prolonged shutdown of the system the temperature of R-23 could probably approcah the ambient temperature.

Why does a liquid receiver give trouble?

for R-23,76 degree F is close to critical point,and the pressure is around 45 bar. you can check it in Coolpack software.
may it give some help.

In my opinion - but who am I - you better use a plate heat exchanger for more than 1 reason:

1. Volume is smaller, so you will have less R23 in the system, so the expansion tank can be smaller.
2. How will you manage oil return? The liquid will be in the lower part of the condenser and the suction is on the top. So the oil will float on the refrigerant and stay in the condenser. The compressor wil lonly pump gasseous refrigerant, no oil.
3. They're much smaller and gives much more capacity for a much smaller space.
4. They're made of stainless steel, so can't rust due to condensation because the condenser will become real cold.

Even if the condensor is well isolated, you still have the R23 evaporator which can become warm during standstill.
You can't afford a pressure release due to an expected pressure rise at stand-still because R23 costs a fortune.

I think you will need an expansion tank of at least 80 litres if you can keep the charge as less as possible.

Will you use a normal fan blown evaporator?

Why not try it - we did it like this in the past - with small service cilinders you connect to the system? If pressure rises too high, connect a second one and a tirth one just till pressure remains at the desired level.
You can use a normal liquid receiver for the final machine.

I have somewhere a formula to calculate the size of the expansion tank. I think it's in a book of Prof Stoecker. The problem with the formula as far I can remember is that you need to calculate the exact amount of gas that will be in the system.

What sounds strange - but it will probably be OK - is the proportion of the 2 compressors: 10 HP versus 40 HP. The systems I worked on - not that much and surely not that big - always had equal sizes. Check perhaps again.

We went with ethylene/propylene till -105°C. I think Bitzer hasn't that much experience with such an application.

This is what I like the most in Refrigeration, going beyond the edges of the normal.

Keep in touch with your progress and make some photos if possible while building your machine.

The 10 H.P. and 40 H.P. is correct. The 40 H.P. is actually a two-stage compressor. Ethylene and Propylene are hydrocarbons and would have different compression characteristics.This could perhaps explain the similarity in H.P.s of low and high side.I suppose Copeland gave you ratings for ethylene/propylene at -100 Deg.C evaporating.I will try Copeland's local partner for details. Are TEVs available for ethylene/propylene?

A good quality oil separator will be installed in the discharge line. If the compression ratios are kept low (I.e. the discharge pressure and temperature is controlled ) not much oil should go past the oil separator.If refrigerant velocities can be kept high enough oil return from evaporator should not be a problem. But with the system (low side) running in vacum, this is a difficult task.

Also this system will have a secondary fluid which will subsequent to chilling be circulated through the jacket of a reactor. So the The evaporator will be either a shell & tube type or PHE. The only reservation I have about PHEs is that I have seen these being not correctly sized many a times. Although the lower volume of R-23 would definitely benefit.In any case I do not intend to use any other material except stainless steel and copper. Rust from mild steel components/pipes would be a taboo for such systems.

In the distant past I can remember working on enviromental chambers at general motors. They fit your description quite well.They were made by an american company called ransco indusries. Would it be an option to buy one ready made or is the cost too different to a home made one? As I remember their chambers ran on the same refrigerants you mentioned but they used carrier compressors. They also had 4 expansion tanks the same size as 60kg refrigerant bottles (in fact I think thats what they were.) They were a bugger though tor condensation and had to be heavilly insulted, and copper used wherever possible to cut down on rust. As peter mentioned they did have water cooled heat exchangers and as I can recall on start up from ambiant the r23 side would cut out 7 or 8 times on the auto hp before it would get away, Ransco informed us this was perfectly normal. The refrigerant charges though were very critical, on these perticular units, the method was to static charge the r23 to 155psi at 21 centigrade ambiant, the 404 was weighed in and topped up at the minus 70 centigrade cycle to fill the liquid sight glass. They ran happily for years the only major problems being the carrier compressors had to be removed to clean the oil filter after the system had been contaminated after a series of heat exchanger failures. (resolved by ransco fitting alternative exchangers) Why dont carrier make their compressor oil filters easy maintenance or is that another story? best wishes dill.

The propylene was used in the high stage (similar to R502) and the ethylene (flammable) was in the low stage.
There are no TEV's for this, so we had to use capillary tubes for this. Load stays +/- always the same with these systems.

For the oil separator try the perhaps a coalescent type.

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they were a bugger though for condensation and had to be heavily insulted

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sorry I meant insulated, although I have called it a few names in the past when unblocking the condensate trays. dill. :p

The expansion tank Peter_1 is refering to is called a "fade-out drum" in the part of the business I am from. Once you have the calculated system volume of liquid and vapor refrigerant, you use the Ideal Gas Laws to get a new volume to fade the entire charge to a safe pressure for the components of your system. Typically you are limited by the compressor working pressure of one of the vessels. You should use 80% of the lowest pressure in your system for the fade-out pressure.

This vessel is typically connected to the high pressure sideof the system with a pressure relief device to let the pressure off as the liquid vaporizes. The vessel is also connected to the compressor suction so that it will pull the vessel back down to design pressure.

If the system is large enough and you will have an extended pull down time you may want to use a compressor suction pressure regulator to limit high suction pressures on the low temperature compressor.

On a smaller system, you might just attach bottles to the system until you reach a suitable settling pressure. Since these fade-out drums can get fairly large, you need to keep the system design such that you minimize volume. The low temperature sides are generally designed as a critically charged system.

Ken

We only used the small cilinders to determine the needed expansion volume. No calculations.
Problem with such a systems is that it's almost not possible to calculate at forehand the critical charge.
We calculated nothing and it still works fine since years now.

Peter, the small cylinders are OK for the small systems. Problem arises if you are designing a large system and you need a fadeout drum that is about 42" in diameter and 20' long. All of my cascade experience has been on the larger sysems...several hundred horsepower total.

With the larger system, you have to take the time to accurately calculate the system volume.

Ken

I have been making auto cascade systems to -160 oC you should be very mindful of two things:
1) The standing pressure and having a large enough expansion tank to cope with start up
2) Oil at low temperatures - the Temprite coalescent oil separators are the way to go

Hi JTSTEN

Welcome to the forum, I hope that both you and us can be of help to each other.

Pls.advise me about where i can find all information of cascade system use IQF .I desire te-60 c , Q = 155 KW.

Now.I can't calculate line size of R 23.Pls. advise me

155 kW with a cascade on R23?
I would love to see this and especially the surge tank for this.
You can calculate the lines manually for every refrigerant.

Now.I have challange job .I would like make cascade condensing offer to customer but I must use BOCK compressor.My point is the exist system of costumer it not work .Pls advise me If I use HGX7/1860-4 (R404a) was 1St. and HAX6/1410-4 ( R23) 2St.Is it correct to pair.The desire point is TE - 60 C ,TC -20 C.I want to know what size of 1.condenser 2.expanse tank 3.expansion valve 4.suctio.line size R23 at 50' ,100' 5.comisioning
Thank you .who help me

Hi I did some quick scale up figures from my systems.
Suction = 4 1/8 to 5 1/8
Discharge = 1/5/8
Liquid = 2 1/8
Condenser would depend on the application. Mine are for freezing water vapour which is removing latent heat and can be quite small. For yours I don`t know just now.
Expansion tank would be around 1 metre cubed. This of course depends on evoporator size and liquid lines.
With reciprocating compressors the flow will need to be about 430m3/h.
Heat exchanger R23 to R507 will need to be around 215KW which is what the R507 system will need to remove.

I welcome any objection to my figures. I am also looking for any help.

If you can have second stage hg series it might be better equipped to take a higher load at start .
D.d.koranne

Thank you for you reply.....Now I am design a new cascade but I use coolpack and duplex software .I am not sure about volume efficiency how many to fill.

Hi
I am trying to design a R23-R22 cascade system. I have decided to have a 6HP scroll Comp for low cycle and 10HP scroll comp for high cycle and a Plate cascade heat exchanger and air cooled condensor. My cooling load is at most 5KW and operating temperatures are -70C for evaporating and 35C for condensing.
I have to have a tight temperature control +/-1C.
I wanted to know if it is ok?!
I don`t have enough catalogues for selecting my compressor, Plate HE, TEV, Solenoid valves,...
whould you please help me?

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Originally Posted by connellpaul

...Suction = 4 1/8 to 5 1/8
Discharge = 1/5/8
Liquid = 2 1/8
...

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You got these figures under what the velocity limits? Would you like to tell us?:confused: