RETA CIRO cheat sheet

The mechanical refrigeration cycle has four steps: compression, condensation, expansion, evaporation; heat is absorbed in the evaporator and rejected in the condenser.

Saturation temperature is the temperature at which a refrigerant boils or condenses at a given pressure; it rises as pressure rises and falls as pressure falls.

Superheat = actual vapor temperature minus saturation temperature at the same pressure. Subcooling = saturation temperature minus actual liquid temperature at the same pressure.

Compression ratio = absolute discharge pressure divided by absolute suction pressure; absolute pressure = gauge pressure plus atmospheric pressure.

Refrigerating effect = enthalpy of vapor leaving the evaporator minus enthalpy of liquid entering the evaporator.

Work of compression = discharge enthalpy minus suction enthalpy; COP = refrigerating effect divided by work of compression.

1 ton of refrigeration = 12,000 Btu/h = 200 Btu/min = 3.517 kW.

Ammonia has a much higher latent heat of vaporization than halocarbon refrigerants, so a given capacity needs a lower refrigerant mass flow.

Ammonia is not miscible with mineral oil, so oil is removed by separators, drains, and oil pots rather than returning with the refrigerant.

Low side: evaporator, suction line, accumulator, and low-pressure receiver up to the compressor suction. High side: discharge line, condenser, receiver, and liquid line.

Single-stage compressors become inefficient and run hot at high compression ratios; low-temperature duty generally uses two-stage or compound systems.

Screw compressors control capacity with a slide valve; reciprocating compressors use cylinder unloading or variable speed.

Pumped liquid recirculation systems use a low-pressure receiver and pump; typical overfeed rates are 2:1 to 4:1.

Thermosyphon oil cooling circulates liquid refrigerant by density difference; liquid injection and water-cooled oil coolers are the alternatives.

Evaporative condensers reject heat by evaporating water in an air stream and require water treatment plus bleed to control scale and biological growth.

Noncondensable gases collect in the condenser and raise head pressure; they are removed by a purger from the highest point of the high side.

Hot gas defrost routes discharge vapor through the evaporator to melt frost and must be followed by a drain-down period before refrigeration resumes.

EPR valves hold minimum evaporator pressure, CPR valves limit compressor suction pressure, and hot gas bypass holds a minimum load on the compressor.

OSHA permissible exposure limit for ammonia is 50 ppm as an 8-hour TWA; NIOSH IDLH is 300 ppm; the odor threshold is well below both values.

Ammonia burns in air at roughly 15 to 28 percent by volume; it is toxic, lighter than air, and rises when released.

Ammonia attacks copper and copper alloys, so ammonia service uses steel, stainless steel, and compatible elastomers, not brass or bronze.

OSHA Process Safety Management applies to ammonia systems containing 10,000 lb or more and requires hazard analysis, written procedures, training, and mechanical integrity.

Before opening any component: pump down or isolate, lock out energy sources, bleed pressure to zero, and verify zero pressure on a gauge.

Never cap, plug, or block a relief valve; relief discharge must be piped to a safe, unrestricted location.

Ammonia emergency PPE: SCBA or supplied-air respirator, full face shield, chemical goggles, impermeable gloves, and protective suit.

Emergency sequence: sound the alarm, evacuate unprotected personnel upwind, isolate or shut down the source, ventilate, and re-enter only with PPE and a buddy.

Hot work on or near refrigeration equipment requires a permit, gas testing before and during the job, and a fire watch.

Ohm's law: V = I × R. Power: P = V × I, also written I²R or V²/R for resistive loads.

Series: total resistance is the sum of the resistances, current is common to all loads, and voltage divides across them.

Parallel: voltage is common to all branches, current divides, and 1/R_total = 1/R1 + 1/R2 + … so total resistance is below the smallest branch.

Peak and RMS: V_peak = 1.414 × V_rms and V_rms = 0.707 × V_peak; a 480 V RMS line has a peak of about 679 V.

Balanced three-phase power: P = √3 × V_line × I_line × power factor.

Synchronous speed N = 120 × f ÷ poles; slip = (N_synchronous − N_actual) ÷ N_synchronous.

A voltmeter is connected in parallel with the load; an ammeter or clamp meter is connected in series with the conductor.

Overload protection guards the motor against sustained overcurrent; fuses and breakers protect conductors against short-circuit and ground-fault current.

Single-phasing a three-phase motor makes the remaining windings draw high current and overheat; phase-loss protection is required.

Power factor = kW ÷ kVA; capacitors supply reactive current and raise power factor.

Ladder rungs are read left (power rail) to right (neutral rail); a coil energizes only when a complete path exists through the rung.

Contacts are named and drawn in the de-energized state: a normally open contact closes when actuated, a normally closed contact opens when actuated.

Start pushbuttons are wired in parallel, stop pushbuttons in series, with the contactor coil in the rung.

A seal-in (holding) contact is wired in parallel with the start pushbutton to keep the coil energized after the button is released.

Safety devices - high-pressure cutout, low-oil switch, overload contact - are wired in series so any one opens the control circuit and stops the machine.

Standard symbols: contactor coil, normally open and normally closed contacts, overload heater, time-delay contacts, and pressure, float, and thermostat switches.

Wire numbers and device designations let you trace a circuit between sheets; the same number identifies the same conductor.