Desuperheaters

It starts condensing. With cold water or too much water flow, the gas is cooled past its saturation point inside the desuperheater and liquid forms in a pack and pipe run that were laid out for vapor only.

The symptoms are unstable head pressure and liquid arriving at the condenser inlet in slugs. Pipe the gas in at the top so any liquid that does form drains forward to the condenser, and control the water side - by flow or by a temperature limit - so the leaving gas keeps some superheat at low load.

For low water flow and a large temperature rise, piped counter-current so the leaving water meets the hottest gas. That gives high water temperature from a modest share of the total heat rejection, which is all the superheat amounts to.

Two limits apply. Slow-moving hard water at high temperature deposits scale, and a brazed unit cannot be opened to remove it. And where the unit carries a potable approval, NSF/ANSI 61 is limited to 90 C. Give us the water quality along with the target temperature.

Look at pulsation and thermal cycling before reordering. The discharge line carries compressor gas pulsation straight into the pack, and pulsation fatigues braze joints. A desuperheater also sees the widest temperature swings in the circuit each time the machine starts against cold water.

Support the discharge piping so the connections carry no load, isolate vibration between compressor and exchanger, and avoid feeding cold water in a sudden step into a hot pack. Then replace against the full nameplate designation; a double-wall unit must be replaced with a double-wall unit. The Double-Wall Separation page explains why.

No. The pressure class has a temperature of its own, and it is lower than the braze figure. M class, the usual refrigerant class, is up to 45 bar at 135 C, and S class is 31 bar at 155 C. A discharge temperature that a copper braze tolerates can still sit above what the class is rated for.

Those are categories, and the stamped rating on the individual unit governs. Where the gas is hotter than a copper-brazed build allows, the nickel-brazed SN construction reaches around 350 C. Send the measured discharge temperature at the worst operating condition.