Economizers

The main liquid flow and its temperature leaving the condenser, the liquid temperature you want at the expansion valve, the refrigerant, and the intermediate pressure at which the side stream boils. That last figure is the one people leave out, and it is not a free choice: it is fixed by the economizer port on the compressor.

Add the superheat the compressor needs on the vapor returning to that port. With those the unit can be rated. Plant capacity alone tells us almost nothing about it.

Nearly always the way the boiling side is fed. The side stream enters as a liquid and vapor mixture, so the unit has to stand vertically with that mixture entering at the bottom and the main liquid running counter-current to it. Laid on its side, or piped so both streams travel the same direction, part of the plate pack sits full of liquid and does nothing while the rest runs dry.

The signs are an expansion valve that never settles, liquid carried over toward the compressor port, and a liquid line warmer than the selection promised. Mounting and Piping covers the port order.

Match the frame, plate count, pressure class and connections, yes. The piping is brazed to the old port positions, and an economizer is small enough that a few plates either way moves the subcooling noticeably.

Read the whole string off the nameplate, including the series letter in front of the frame number. A V80 and a B80 look the same on the bench, but only one has a distribution device on the boiling side. If the plate has corroded past reading, send the outside dimensions, the port centres and the refrigerant, and we will work back to the build.

Treat the whole unit as high pressure. The class letter applies to the exchanger, not to one circuit, and on CO2 the side that runs at intermediate pressure in operation can climb well above it when the plant stands idle and warms up. The U class covers 64 to 140 bar, which is why the B4T and B18 are the frames used here.

Where UL approval is required for CO2 duty, plan on a relief valve on each side set at 0.9 times design pressure. Leaving the valve off the lower-pressure side is the usual omission. See Pressure Classes for the other class letters.