Refrigeration Design

[johnemar]

Hi Guys,

I am atfer some input regarding a refrigeration design that one of our "electronic enginners" has decided to come up with and any feedback on wether it will work OK would be appreciated.

OK, we Have 2 x condensors that he has piped in parrallel, (not water cooled) 1.9 kw each and a compressor of 4kw.

He has gone from the discharge pipe into 3/8s 0.7m up he has Tee'd off into a 5/8s pipe,( Note this is a 90degree bend in both directions) 6 inches from the tee "left and right" he has then reduced the 5/8s into 1/4" pipe, and they run into the condensors.

There is no flow control valve in the line

My question is would he be better of pipeing the system in series, giving better sub cooling and less flashing off, or would his system be best.

Thanks in advance for any replies.

BR John M

[Andy]

Hi johnemar,
the reason for piping in parallel is to reduce pressure drop through the condensers. I have a friend who piped two condenser such as that in series, that was a doubling up of surface area in a warm roof space, he says it worked alright, but the pressure loss through the condenser was a little high.
You could pipe two condensers in series, but the tubes would have to be sized to reduce the pressure drop.
Regards Andy

[Gary]

Piping the two condensers in series would cause an extreme restriction.

Even piping two of them them in parallel is borderline. You are trying to squeeze the output of a 3/8 line through two 1/4 inch lines. But since the combined condenser ratings equal the compressor output rating, it will probably be okay.

I would minimize the use of 1/4 tubing, taking 3/8 right up to the condensers.

[Dan]

The subcooling and pressure drop are related. Or am I wrong? Why are we assuming that series piping the condensers will achieve better subcooling?

[johnemar]

Hi Guys,

Thanks for all of your comments.

Gary:
Can you please elaberate on why either piping could cause restrictions.

Dan:
We are assuming that we would get better sub cooling as we have had this unit out in the field for over 1 year with only 1 x 1.9kw condensor and the reports we have been getting back is that the unit has been going out on HP when a load has been placed on it.

Placing a second condensor in their will hopefully help get rid of the heat that is causing the problem, and this will give us subcooling.

Note this unit isnt your normal system it is a speciel effects unit that you blow smoke through and get a low lying smoke, ( like you see if films or in the theatre) and when you running under normal conditions without load it runs at around 200psi, but whan a load is placed on it it can go upto 350-400psi in high ambient temps and the HP switch is ratted to 400 psi.

Thanks again all!!!!

[herefishy]

Oh, so this activity is to correct an existing problem. Before going through all this trouble, I would first be checking the compressor manufacturer's design operating range. I see many applications in which a 35F refrigerated space is being served by a condensing unit (compressor) rated at a maximum evaporating temperature of 25F. At a 10F T.D., the maximum S.S.T. is reached whenever.... THE UNIT TURNS OFF!!!

I like to employ equipment with at least a maximum evaporator temperature rating of 10F above the design S.S.T. This helps pull-down, and assures equipment life. The reason you find such applications is because the low-bidder gets the job, and the smaller C.U. that is getting max'd out is cheaper, than one with an operating range such as that I suggest.

In the case of some R-12 retrofits, I have encountered a particular manufacturers equipment which is not conducive to the proper employment of R-409A. If the existing equipment is a retrofit, particular with a (blend) of higher saturation pressures, you may find that even if you call the (compressor) mfgr. for advice, they might say, "Don't do that!".

[johnemar]

Thanks Herefishy,

This is exactly what I told the "Electronic engineer" 16 months ago when he started to build the original product. But unfortunately he is one of these guys who doesn’t listen to any one but himself. (He’s a bit of a know it all)

Now we have had to build a MK2 and he decided to still use the same size compressor and upgrade the condenser.

Only time will tell

Br
John M

[Gary]

What are the subcooling (condenser outlet) and superheat (compressor inlet) temperatures with full load?

Also what are the air temperatures entering and leaving the condenser?

[Gary]

Gary:
Can you please elaberate on why either piping could cause restrictions.

When you pipe a big line into a small line you get a restriction. That's how a cap tube works as an example. When you put those small lines in series you increase the restriction. Think long cap tube.

When you pipe two lines in parallel, each has to handle only half the flow, so there is less restriction.