Few Points on Ammonia systems

[smpsmp45]

Pl. see the File

[Brian_UK]

Welcome to the forum, it is easier for people to answer questions if it is shown directly. Having to open another file is uneccessary.

This is the content of the pdf file from the above post.

One of our customers has Installed Total TEN Plate freezers at their New Factory. They are looking for some solutions to following points. There is a TWo Stage Recip Compressor System with -32 Deg C as evaporating tempeartures. The Liquid at -32 Deg is pupm circulated in the Plate freezers.

The Indian manufacturer has provided Orifiice plates for Each Plate freezer plate. What is the purpose of 2.5 mm orifices at the liquid entry of each plate ? especially when we are circulating 10:1 ratio through freezers in pump circulation ammonia systems.
What is acceptable core temperature difference between top plate product and bottom plate product with 90 min cycle and -32 deg c liquid. Product loading temp. 20 deg c.

What is standard practice to adjust liquid ammonia flow through each freeer with the help of regulating valves in inlet lines.Should it be based on min pressure drop or min temp drop between inlet and outlet to and from freezer

In order to have a same Liquid flow through each plate freezer, we have provided Flow Control Valves . But the point is how to adjust the Flow?
Can orifices be removed for pump system what purpose do they serve besides resrticting liquid flow.

If orifices are must should they not be of different diameters , smaller for lower plate and increaseing up to upper plate so that lquid flow balance is maintained

[smpsmp45]

Your point is noted. Next time I shall be careful

[Peter_1]

Brian, I tried to do this myself at least 3 times for smpsmpsmpsmp but the server returned each time a fault message.

[nh3wizard]

My first question would be "What does the manufacturer of the plate freezer recommend on how to adjust?" Or what size orifices should be installed.

[smpsmp45]

The issue here is the Plate Freezer manufacturer recommends 2.5mm dia hole orifices.

Now what we have done is that, we have adjusted the flow rates equal in all the plate freezers by loading all the freezers & taking the pr. drop across the each plate freezer. This was now we are getting temp drop of about 1 deg c between the bottom plates & the top plates ( the plate freezer is bopttom feed type). We first removed the orifices, but later again reinstalled them.

But this is only trial & error method &going in for text books & infact we had a good standards published by US marine dept & that has helped us a lot.

But the point is why 10: 1 ratio. We checked various plate freezer manufacturers & no one goes beyond 4: ratio.

So practical inputs are required.

Many thanks.

[US Iceman]

At a 10:1 ratio you may only be pumping 100% liquid through the coil and it may not be boiling. I'm not sure what the reasoning is for the 10:1 ratio. You are right about the normal being 4:1.

The orifice has to be sized to provide sufficient pressure loss to bring the liquid pressure (being pumped into the coil) down to the evaporating pressure in the coil.

If the liquid pressure downstream of the orifice is greater than the evaporating pressure the liquid will not boil and only picks up sensible heat.

Without boiling there is no phase change, and this is how you achieve better cooling.

[Andy P]

At a 10:1 ratio you may only be pumping 100% liquid through the coil and it may not be boiling. I'm not sure what the reasoning is for the 10:1 ratio. You are right about the normal being 4:1.

The orifice has to be sized to provide sufficient pressure loss to bring the liquid pressure (being pumped into the coil) down to the evaporating pressure in the coil.

If the liquid pressure downstream of the orifice is greater than the evaporating pressure the liquid will not boil and only picks up sensible heat.

Without boiling there is no phase change, and this is how you achieve better cooling.

Ah yes, but....

It depends on the size of each plate and the maximum size of the suction hose connection that can be fitted onto the plate. If the plate heat load is relatively large then there will be a big pressure drop in the hose if you try to put gas through it. Increasing the circulation rate (for example from x4 to x10) in these circumstances means the liquid doesn't boil so the pressure drop is less in the hose. This means that for a given compressor suction condition the plate temperature is actually lower without evaporation . The refrigerant is being used as a brine and the gas for the compressor suction is created as flash gas in the hose and suction header.

I suspect that the orifice is not there to restrict flow during refrigeration; it prevents too much of a good thing during hot gas defrost, ensuring that all the plates defrost correctly. Remove it at your peril!! (What I mean is don't remove it if you want the plates to work properly)

cheers

Andy P

[Josip]

Hi, smpsmp45

usually PF are working with evaporative temperature at -35C/-40C and you have -32C...

maybe some questions:

This was now we are getting temp drop of about 1 deg c between the bottom plates & the top plates ( the plate freezer is bopttom feed type).

I believe freezers are horizontal type with vertical liquid header and thats why they have orifices, but do not understand what is bottom feed type possible some sketch/photo?

Circulation ratio 10:1 is calculated when all (10PF) are working or...seems too big..

Anyhow, it is not possible/practical to work with all PF simultaneously (maybe I am not right?) ...i.e. some of them are under loading, some working, some defrosting and some unloading...to have continuous process...

Did you try to do complete process within one maybe two PF?

Ok, maybe you need to adjust liquid ammonia pressure (open by-pass valve within pump station or close discharge valve on the pump to reduce flow (US Iceman mentioned this in another post http://www.refrigeration-engineer.co...61&postcount=6)

Best regards, Josip

[US Iceman]

This means that for a given Compressor suction condition the plate temperature is actually lower without evaporation .

I'm thinking the temperature difference would only be due to the offset in the reduced pressure loss (hoses and fittings) with the brining versus boiling. Is that what you are describing Andy?

Interesting...

[Andy P]

I agree with Josip - I am having difficulty visualising this. It does sound to me like a horizontal plate with a vertical header, otherwise the discussion about top and bottom plates doesn't make sense. Do all the plates defrost at the same time? If so, as I said earlier, the orifice is there to stop all the gas rushing through the first plate to defrost, leaving the others stuck. Don't take them out.

I have heard of plate freezers in Australia that needed x15 to work right. Even at x4 remember that 3/4 of the liquid is not evaporated.

If the suction of the ammonia compressor is at -32 degC but there is a 0.65 bar pressure drop in the hoses then the plate internal temperature is only at -22 deg C. Increasing the flow rate in order to suppress boiling might reduce the pressure loss by 0.5 bar (because there's now no gas in the hose), with a bit of a temperature rise in the liquid, say 5K. With the higher flowrate the ammonia now rises from -29 deg C to -24 deg C, whereas before it was evaporating at -22 degC.

cheers

Andy P

[smpsmp45]

I shall try & describe the total System once again separately. But one thing for sure, that Flow rate as specified by the manufcturer is definitely on the very high side. We also did some trials as ratio specified by other companies & other equipment manufacturers was 4:1 or 3:1.

So we bypassed liquid & tried to maintain lower flow rate. With that the situation improved a lot.

Orifices are still required for sure.

The PF are Horizontal type but I shall collect exact details & send a more compiled version of the scene.

Many thanks for studied replies by all

[Gonzalo Arias]

I think that the temperature difference between the top and bottom plates is due to the liquid head pressure, and you can’t do any thing to help it.
Good luck

[smpsmp45]

It was a great pleasyre reading all the replies. Based on these points, we could set right the freezers. yes they are operating with a 20 minute differential - loading time. & hence with 90 minute freezing cycle, we had almost 10 min opverlap for the fifth PF.

We reinstalled all the Orifices, we adjusted the flow rates to have 4:1 ratio only. ( this point we had a tough time with the client though).

But now the system is steady & is performing as per design parameters. Now the issue of setting the evaporating to -40 degc, also has been discussed with the client & luckily, we had provided all the sizing to suit -40 evaporating ( if the need arises) & though we are runining the plant at -32 at present, may be after a month or so, till the client gets confidence on all the issues, we shall change over to -40 degc evaporating ( we have to change the flywheel sets etc.)
But yes, the Orifice plates are almost essential. One can ot remove them. We adjusted the flow rates using pr. differential & not the temp differential.
The temp differrence between the top & the bottom plate products is now almost 2 /3 deg c.
That now we are tackling by loading the top side & coming to the bottom plates. So the Top plates get more time. & at the time of removal after freezing we are removing the bottom product first going towards top.

many many thanks

[ntfreezer]

Now what my factory has done is that, we product a lot of plate freezer, we change orifice in order to dimension of evaporate plate :the orifice of horizontal plate dimension 1200mmx2100mm is 3.5mm, the orifice of horizontal plate dimension 1000mmx600mm is 2mm, adjust the flow rates equal in all the plate freezers .
the ratio is normal4:1.

[smpsmp45]

This is very good. After lots of exchange of ideas, this company is picking up the right points in the discusisons & implementing those. I think this is the best use of this forum