RETA CIRO common mistakes
RETA CIRO · Understand the traps and how to avoid them.
Entering gauge pressure directly into a saturation table or PT chart when calculating saturation temperature, compression ratio, or superheat.
Gauges on an industrial ammonia system read psig, but refrigerant tables are built on absolute pressure. Under exam time pressure the conversion step gets skipped because the gauge number 'looks like' the table number.
What to do instead
Make the conversion a written step, not a mental one: every time a pressure leaves a gauge and goes into a table, write 'psig + 14.7 = psia' on the scratch sheet and label the units on the answer. Sanity-check with a known point, e.g. ammonia at 0 psig is about -28 F, not 0 F.
Measuring superheat at the wrong location (e.g. at the compressor suction flange and calling it evaporator superheat) or reading subcooling on the wrong side of the condenser.
Suction line heat gain between the evaporator outlet and the compressor makes the two numbers different, so the concept blurs when the question only says 'superheat' without a location.
What to do instead
Anchor each term to a physical point before answering: evaporator superheat = suction temperature at the evaporator outlet minus saturation temperature at that same pressure; total superheat = compressor suction temperature minus the same saturation temperature. If the item doesn't name a point, state the point you are assuming in your working.
Choosing 'capacity increases' when suction pressure is lowered, or 'compression ratio decreases' when condensing pressure rises.
Candidates reason from compressor displacement (a fixed number) instead of from the evaporator temperature difference and the pressure ratio, so direction of change gets reversed.
What to do instead
Use two one-line rules on the scratch sheet: lower suction pressure = larger TD across the coil but lower refrigerant density at the compressor = less capacity per unit volume and a higher compression ratio; higher condensing pressure = higher compression ratio, higher discharge temperature, lower capacity. Apply the rule mechanically before looking at the options.
Setting two-stage or compound-system interstage pressure at the arithmetic midpoint of suction and discharge, and then sizing the interstage load as only the low-stage mass flow.
The 'middle' of a pressure range feels intuitive, and the desuperheating load from the high-stage discharge into the intercooler is easy to forget because it isn't a refrigeration load in the usual sense.
What to do instead
Compute interstage pressure as approximately the geometric mean (square root of suction psia times discharge psia) and check it against the option set. For interstage load, add the desuperheat duty to the high-stage flow and note in your working which fluid streams cross the intercooler.
Treating the recirculation ratio as if it were the pump's flow rate, or assuming a higher recirculation rate always improves evaporator performance.
The ratio (mass of liquid circulated per mass evaporated) and the actual volumetric pump flow are related by liquid density and load, and candidates collapse the two into one number.
What to do instead
Separate the two calculations explicitly: recirculation ratio = total liquid circulated / liquid evaporated; pump flow = evaporator duty converted to mass flow times the ratio, then to volume using liquid density at the pump temperature. Remember that beyond the point where the coil is fully wetted, extra recirculation adds pump power and liquid return risk rather than capacity.
Sequencing hot-gas defrost incorrectly, e.g. admitting hot gas to the evaporator while the suction is still open to the low side, or skipping the pump-out of liquid before defrost begins.
The steps are learned as a list rather than as a reason-driven sequence, so under exam pressure the order is reconstructed by guesswork.
What to do instead
Derive the order from what each step protects: stop liquid feed, pump out or drain the coil so liquid doesn't have to be boiled off against the low side, isolate or re-route suction, then admit hot gas, then after defrost re-establish refrigeration through a controlled re-cooling/pull-down before returning to normal feed. Rehearse the sequence by naming the hazard each step removes.
Confusing the ammonia exposure numbers, e.g. quoting 300 ppm as a threshold limit value rather than the IDLH, or treating the PEL/TLV as a level at which a full-face respirator is unnecessary.
Several numbers circulate for ammonia (TLV, STEL, PEL, IDLH) and they are all in the tens-to-hundreds of ppm, so they get swapped under recall pressure.
What to do instead
Memorize a small set with labels attached and never quote a number without its label: ACGIH TLV 25 ppm TWA with a 35 ppm STEL, OSHA PEL 50 ppm TWA, IDLH 300 ppm. Practice writing the whole labelled table from memory, then answer exposure items only from that table.
Describing equipment isolation as 'close the suction and discharge valves' without applying lockout/tagout, proving zero energy, or accounting for trapped liquid.
Valve closure is the visible action and LOTO is administrative, so the written answer stops at the visible step and misses trapped refrigerant, stored energy, and restart risk.
What to do instead
Answer isolation items with a fixed four-part structure: notify, isolate (valves and electrical), lock and tag, then verify/bleed to zero energy and check for trapped liquid that can build hydraulic pressure. If the item asks for a procedure, give all four parts even if only one is scored.
Adding resistances directly for resistors in parallel, or assuming total resistance in a parallel circuit is larger than the largest branch resistor.
Series rules are learned first and dominate recall, and parallel circuits are often drawn in a layout that hides the branches.
What to do instead
Use the reciprocal formula (1/RT = 1/R1 + 1/R2 + ...) and always apply the bounding check: total parallel resistance is less than the smallest branch resistor. Verifying against that check takes seconds and catches the error before you commit to an answer.
Reading ladder diagram contacts in their actuated state instead of the state drawn, or tracing a rung from the load back to the neutral instead of from the energized rail through the logic to the load.
Symbols are drawn in the de-energized condition, but candidates mentally 'run' the machine and read a normally-closed contact as open because they know the device will be energized in service.
What to do instead
State two things before answering: 'drawn de-energized' and 'power flows from the live rail, through the contacts, to the load, then back on the neutral.' Trace forward from the rail on paper, mark each contact as conducting or not in the de-energized state, then re-evaluate rung by rung for the energized condition the question asks about.