Regulation regarding hydrocarbons in an cascade

[DaBit]

I am currently in the process of building a cascade for my PC. The cascade will run with R507 using a Tecumseh CAJ2464Z 1.5ph R404a LBP compressor for the high stage, and with ethylene (R1150) using a Danfoss SC21B LBP R502 compressor for the low stage.

Thus, the low stage is going to run using 'feared hydrocarbons', and one which is capable of developing extreme pressures when all active and passive safety measures fail (over 50 bar is possible).

Now, I was wondering what the official requirements (preferrably Dutch/European) are for the low stage. High side test pressure, low side test pressure, special precautions, etcetera.

Ethylene scares me, so everything will be built to survive an atomic war. Therefore I guess that it takes little effort to have the system adhere to the standards, so I would prefer to do so.

[Peter_1]

We service some applications with ethylene.

You have to install an expansion vessel on the low side for the ethylene compressor so that the ethylene can expand to normal standstill pressures (let's say 15 bar)

How to dimension this vessel? I should not know it. We rebuild some machines years ago and we did it we the trial and error technique. But we had examples. I should say, start with +/- 3 to 5 l., a liquid receiver you have somewhere, an empty gas cylinder or the little gas cylinders from Campingaz.

The compressor manufacturers has always the correct figures for max allowed LP at standstill (mostly 20 bar)

You don't have to be afraid of the ethylene in my opinion, it's almost like propane and the quantities that you will need are so little that I don't see any problems concerning the used gas.

Requirements are that you must do a pressure test at 1,3 x max working pressure but you can never go higher then the max allowed pressure.
You also have to do a leak test.
This has to be done by a STEK certified technician but that did you knew already.

We pressurize always at 25 bar with dry nitrogen.

I think you know how everything must connected electrically. Use as much soldered equipment (driers, pressosotats,...) to minimize leaks.

[DaBit]

Originally posted by Peter_1
You have to install an expansion vessel on the low side for the ethylene compressor so that the ethylene can expand to normal standstill pressures (let's say 15 bar)

I am aware of that.


How to dimension this vessel? I should not know it. We rebuild some machines years ago and we did it we the trial and error technique.

An idea of the size can be obtained by determining refrigerant densities at the various locations in the system, and adding them. You know where liquid must be, where high pressure gas is, and where low pressure gas is. This gives you a rough estimate of the refrigerant charge required.

Next: determine the volume you need to contain that amount of gas at designed static pressure. Subtract total internal volume of the system, and you have the estimate of the expansion vessel required.

This is also why I try to keep high side volume as low as possible. The higher the high side volume to low side volume ratio is, the larger the expansion vessel required. If there is ample high side volume and only liquid in front of the captube, the compressor shell provides enough volume to limit static pressures.


But we had examples. I should say, start with +/- 3 to 5 l., a liquid receiver you have somewhere, an empty gas cylinder or the little gas cylinders from Campingaz.

An old liquid receiver connected to the system by means of a piece of capillary tube will do.


The compressor manufacturers has always the correct figures for max allowed LP at standstill (mostly 20 bar)

I would prefer a lower static pressure. It's hard on the compressor to start with 20 bar backpressure.


You don't have to be afraid of the ethylene in my opinion, it's almost like propane and the quantities that you will need are so little that I don't see any problems concerning the used gas.

True, but there are complications. The evaporator head ends in an airtight housing filled with dry air. A small leak causes the ethylene to accumulate inside, possibly raising ethylene concentration between LEL and UEL. A situation that must be avoided.


Requirements are that you must do a pressure test at 1,3 x max working pressure but you can never go higher then the max allowed pressure.
You also have to do a leak test.
This has to be done by a STEK certified technician but that did you knew already.

No problem. High stage is leak/pressure tested by a STEK-certified technician already. 10 bar low side, 25 bar high side.


I think you know how everything must connected electrically. Use as much soldered equipment (driers, pressosotats,...) to minimize leaks.

Everything will be brazed, even the (braided stainless steel) hose to the evaporator. The only exception is the oil separator, which is equipped with flares. But I have a few of those nice STEK flare to solder adapters with copper rings in them. They will do...

[Peter_1]

As far as I understand, you already had the answers for your questions :)

Connection between expansion tank and LP can be with 1/4 - 3/8 tube. I don't see why a capillary tube.

I know you can calculate it - we did it in the past and you can calculate anything - but there will always remain things you can't calculate correct.
Therefore, the practical setup like we did is far more efficient (we connected several small cylinders through our common manifold)
In business is time money you know.

You had already the answer to prevent excess pressures with ethylene --> Expansion tank is the only solution.

Equilibrium pressure in our systems is 3,4 to 3,8 bar.

If there is ample high side volume and only liquid in front of the captube, the compressor shell provides enough volume to limit static pressures.
I should not count on that but practical tests will tell you more.

You asked for test pressures: you're now facing the new PED regulations.
http://ped.eurodyn.com/

[DaBit]

Originally posted by Peter_1
[B]As far as I understand, you had already the answers for your questions :)

Oh, I know how to build a cascade. At least in theory :)

But what I don't know is what the official requirements regarding safety etc. are.


Connection between expansion tank and LP can be with 1/4 - 3/8 tube. I don't see why a capillary tube.

Only a few of which only the last one really important.:
- Lower heat loss through the pipe
- Easier to route (even easier than 1/4")
- smoother feed of refrigerant to the compressor during startup. This reduces peak load on the high stage during startup of the low, allowing me to choose a TEV orifice for nominal load instead of peak load.


but there will always remain things you can't calculate correct.

Of course, but I'd like to know if I need nothing, 1 dm³, 10 dm³ or 100 dm³. Math is a good tool to retrieve this information. Of course I cannot calculate that I need 3.47621 dm³ to get static pressure at exactly 5.0000 bar, and expect it to be like that in a real system.


I should not count on that but practical tests will tell you more.

Indeed :)



You asked for test pressures: you're now facing the new PED regulations.

I will read through that site. Let's see if it's useful.