Undersized means the compressors picked for this group can't move the mass flow the cycle puts through it. On a transcritical booster's medium temp pack, or a two-stage system's HP stage, that flow is well above the medium temp evaporators' own.
Why Kelvin judges it this way
These compressors lift more than the vapour from the medium temp evaporators:
- A booster's medium temp pack also takes the low temp compressors' discharge and, with the flash gas bypass valve (FGBV → MT suction), the receiver's flash gas.
- A two-stage HP stage takes everything the flash tank sends up as well: the low temp discharge, the vapour boiled off cooling it, and the flash gas. It draws at the interstage.
So on Equipment, under Compressors, these groups are judged in kg/h against the cycle's own mass flow, not in kW against the medium temp cooling load. The line under the group's heading gives the flow part by part, and the state the compressors draw it at. As the group's rule says, under 0% spare is undersized and over 50% spare is oversized.
What to do
- 1
Read the line under the Medium temp suction group heading, starting “Judged on the mass flow the pack carries” or “Judged on the mass flow the HP stage carries”, to see what makes up the flow.
- 2
Pick compressors for that flow: more units, or larger models, until the group's capacity in kg/h covers it.
- 3
On a booster, routing the flash vapour to a parallel compressor takes the flash gas off the medium temp pack. On Cycle, under Flash vapor routing, choose HT suction group, save the design, and the parallel compressors are judged on the flash gas in a group of their own.
- 4
A compressor entered as a rated point isn't judged on a mass-flow group. Enter its displacement, or coefficients that include a mass-flow set.
More on that last case is in Why does an equipment pick read Not judged?.