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..?