Return double risers in overfeed (pumped) refrigeration system

Hi All,

Recently we came across a nasty problem with a blast freezer. A brief system description is,

System : Liquid over feed (5:1)
R717
-45°C
blast freezer

Blast freezer is located on the ground floor and return header is located at around 12m from the ground. So we used double riser in the wet return line.

When we commissioned the plant, the blast freezer worked perfectly during startup, but towards the end of batch freezing the evaporation temperature changed drastically, increased almost 5 degrees.

We checked all the piping filters, setpoints and repeated the process and we got exact same result in the second run. Now we suspect the double riser could be the problem. Towards the end of batch freezing, ammonia evaporation will be low and there will be less gas in the wet return line. Double risers cause this gap to escape through the small pipe resulting liquid ammonia to get collected in the big pipe and the pressure exerted by the static column messes up the evaporation temperature in the coil.

I don't know if I explain the situation well, anybody faced anything like this before..?. Is double risers a good practice specially in very low temperature overfeed systems..?

Rocky,
12 metres you may need a double, double riser, or triple riser, depending on maximum & minimum loads.
Yes good practice & works if designed well.
Would say regulating valve has to be set accurately.
If Danfoss can work out setting off Coolselector.
Maybe at some point liquid solenoid could be cycled on/off to not over flood evaporator & have column of liquid when low load (experimental).

hello Rocky123,
if you run coolselector from Danfoss you can see that pressure drop in vertical return lines are important at so low temperature. tried here with 500kw.....
bigger line, worse result in vertical.
for me 5 is a high recirculation rate, I have done recently flow measurements on a blast freezer where there was some ''complain'', found a rate of 15, it works better with the requested rate of 3!!


Attachment 15962Attachment 15963

Can you give us the drawing of your risers? I know one plant where double riser was install but it was done wrong way. You suction line 12 meter from the ground. How many meters you should lift the liquid form the top of the evaporator to the suction line?

This is an option for precise control in mentioned system.
Very reputable company.
Not cheap, but works.

https://www.hbproducts.dk/en/product...rainer-housing

Hello ranger,
We tried to cycle liquid solenoid valve on/off, there is a difference and we are trying to optimize it.
Are double risers doing more harm than good in pumped overfeed systems...?
Main aim of vertical recirculated suction piping is to make sure flow separation won’t occur between gas and liquid phases. By providing double risers, aren’t we creating the situation for flow separation in vertical piping..?

Hello segei,

We did double riser as per the attached schematic(from ashrae refrigeration). Small riser sized for 1/3rd of the capacity and big pipe sized for 2/3rd of the capacity. From top of evaporator to suction line is around 9 meter.

Attachment 15964

Hello cricri,

Yes, coolselector is a very useful tool and keeps on improving every week, Now it even plots min required riser mass flow rate in vertical pipes.

But for batch freezing applications, towards the end of cooling cycle, evaporation will be low but still we pump the same amount of liquid into the coil(circulation ratio changes). Then the return will have less vapor and density increases a lot and create a big static penalty.

Quote:

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Originally Posted by ROCKY123

Hello ranger,
We tried to cycle liquid solenoid valve on/off, there is a difference and we are trying to optimize it.
Are double risers doing more harm than good in pumped overfeed systems...?
Main aim of vertical recirculated suction piping is to make sure flow separation won’t occur between gas and liquid phases. By providing double risers, aren’t we creating the situation for flow separation in vertical piping..?

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Rocky,
There must be a maximum height for double riser before it stalls.
Does ASHRE recommend?
You say it mentions 1/3 capacity, maybe that’s to much.
If you calculate load when low, that would be more realistic.
It is possible to have triple riser.
Double riser works, liquid sticks to pipe, gas goes up middle of pipe.

Air on & off could help control solenoid.

Did you heck regvalve setting on oil selector?