11.1 Using CIRO Operating Screens (normal vs abnormal)

11.1 Using CIRO Operating Screens (Normal vs Abnormal)

The January 2023 CIRO Study Guide is blunt about why the exam puts operating screens on the glass. You use them to (1) determine refrigerant condition at a location, (2) diagnose and adjust, (3) compute cost in dollars per hour and energy over time, and (4) use property tables. The same sitting also gives you formulas, theoretical discharge-temperature charts, electrical diagrams, an SDS, saturated-property tables, and 24 troubleshooting screens. Those 24 screens are the monitoring drill: each view is NORMAL or ABNORMAL. The interface does not tell you how long that picture has been true. It could be ten minutes old. It could have been that way for months. Your job is to interpret the state points, not to invent a timeline the exam did not print.

Do not use CTRL+F on the on-screen PDF. Learn where the formula sheet, saturation table, discharge chart, SDS headings, and troubleshooting set live so you can scroll under the clock.

What NORMAL and ABNORMAL actually mean

A normal screen is a snapshot whose numbers agree with each other: suction pressure and suction temperature produce a believable superheat for that system type; discharge pressure matches a condensing temperature that the sump, wet bulb, and liquid temperature can support; oil differential and oil temperatures show lubrication and cooling; slide-valve percent and the ammeter tell the same loading story. An abnormal screen is a snapshot where one or more of those relationships has broken. RETA will not stamp "dirty condenser since March." It will give you a discharge pressure that no longer matches the wet-bulb story, an oil ΔP that no longer matches a clean filter, or a suction temperature that is wet or wildly superheated.

Because elapsed time is missing, do not wait for a trend arrow on the exam item. Compare live values to saturation, to the theoretical discharge chart, and to each other. In the plant, that same habit is why you keep a KPI log. The exam's missing timestamp is the real world's slow failure.

Method: refrigerant condition at a location

Every pressure/temperature pair is a state point. Convert gauge pressure to absolute unless the table is already in psig:

psia = psig + 14.7 (use another barometer only if the stem gives one).

Look up saturation temperature at that pressure. Then:

• If measured temperature is above saturation on a vapor line, the refrigerant is superheated vapor. Superheat = T_measured − T_sat.
• If measured temperature is below saturation on a liquid line, the refrigerant is subcooled liquid. Subcooling = T_sat − T_liquid.
• If a vapor line sits on saturation (T ≈ T_sat), the stream may be saturated vapor or two-phase — a liquid-carryover threat at a screw suction.
• If a "liquid" temperature is at or above T_sat, you do not have subcooled liquid. You may have flash, a bad sensor, or a condenser that is not rejecting heat.

Use one table for the whole item. Mixing a remembered "20°F is about 33 psig" with a psia row is how people name the wrong phase.

Representative ammonia saturation values (the same family of rows you will see on screen):

How to read a 300 HP-class screw screen

Industrial CIRO-style screens for a large rotary screw commonly show 480 VAC, a power factor in the mid-0.8s, a motor efficiency in the low-to-mid 90s percent, thermosiphon oil cooling, and an evaporative condenser. Those are typical package facts. They are not a license to memorize a copyrighted sample question list. Invent nothing about RETA's wording. Walk this screen in a fixed order so you do not skip the field that is actually abnormal.

1. Electrical identity. Confirm voltage is line-to-line three-phase. Read amps, PF, and efficiency. Compute input kilowatts even if kW is already displayed — matching the displayed kW is how you prove you kept PF in the formula.

kW = √3 × V × I × PF / 1000 with √3 ≈ 1.732.

2. Slide-valve percent. A screw at 100% with low amps is unloaded some other way (VFD, suction throttle, or a lying transmitter). A slide at 40% at near-FLA is a different lie. Slide percent is not percent of kW and not percent of tons.

3. Suction pressure and suction temperature. Convert P to T_sat. Compute suction superheat at the compressor. A flooded or overfeed plant feeding a vessel should show modest superheat at the screw — enough to prove vapor, not a DX-style 20°F hunt. Near-zero or negative superheat is a slug threat. Large superheat is a starved evaporator, iced coil, closed suction stop, or a heat source on the line.

4. Discharge pressure and discharge temperature. Convert discharge P to condensing T_sat. Compare actual discharge temperature to the theoretical discharge chart on the correct PSIG or PSIA scale. Oil-injected screws run cooler than a dry reciprocating curve; a jump versus the chart at the same compression ratio is still a fault.

5. Oil ΔP and oil temperatures. Plants label "oil ΔP" two ways. Filter ΔP is across the oil filter (clean often in single-digit psi; OEM change-out is commonly in the mid-teens to around 20 psi — use the package). Lubrication ΔP is injection or separator pressure minus suction (or the OEM reference). The oil-pressure interlock uses lubrication differential after the start delay. Thermosiphon: oil to the cooler is hot; oil from the cooler (supply/injection) should be clearly cooler. Hot supply oil plus high discharge temperature is a cooler-flow problem until proven otherwise.

6. Condenser sump and condensed-liquid temperature. Condensing T_sat minus leaving sump water is a condenser TD. Condensing T_sat minus entering wet bulb is evaporative-condenser approach. Liquid below T_sat is subcooling. High head with a modest wet bulb and a wide approach is dirt, scale, failed fans or spray, or noncondensables — Chapter 12 carries the full diagnostic; this section only requires you to read those fields.

Worked screen — West screw, 300 HP class (original values)

This is a constructed operator round, not a reproduction of RETA items.

Suction state. 19.6 psig + 14.7 = 34.3 psia → table 5°F saturation. Measured 11°F → 6°F suction superheat. The compressor is seeing superheated vapor, not liquid. That is a normal vessel-fed screw picture.

Discharge / condenser state. 151.7 psig = 166.4 psia → 85°F condensing. Discharge gas at 162°F is 77°F above saturation — compare that gap to the theoretical chart at this ratio. Condensed liquid at 80°F → 5°F subcooling. Sump 77°F → condenser TD = 85 − 77 = 8°F. Approach to wet bulb = 85 − 67 = 18°F — not a trophy, not yet the 25–30°F "this condenser is in trouble" band.

Oil. Filter ΔP 9 psi is a clean-filter story. Supply 116°F with separator 152°F means the thermosiphon is removing heat.

Electrical. kW = 1.732 × 480 × 328 × 0.88 / 1000. 1.732 × 480 = 831.36; × 328 = 272,686; × 0.88 = 239,964; ÷ 1000 = 240.0 kW. Shaft HP ≈ input kW × efficiency / 0.746 = 240.0 × 0.94 / 0.746 = 302 HP. The 300 HP nameplate is motor class, not a measured shaft load; here you are essentially at class, which matches 328 A on a motor whose 460 V table FLA is on the order of 361 A.

Compression ratio in psia: 166.4 / 34.3 = 4.85. Gauge division 151.7 / 19.6 = 7.7 is not a ratio.

At a typical industrial energy price of $0.17/kWh (the figure CIRO practice prompts use — treat it as an example rate, not a published tariff): 240.0 × 0.17 = $40.80/hr. Section 11.3 extends that hour into a day, week, and 5,000-hour year.

The same machine, abnormal — still no elapsed time

A second screen — the abnormal member of a troubleshooting pair — shows the same West screw:

181.1 psig = 195.8 psia → 95°F condensing. Approach = 95 − 67 = 28°F. TD = 95 − 86 = 9°F. Subcooling = 95 − 92 = 3°F. Head rose 10°F of saturation while wet bulb did not. That is not "a hot day" if WB is still 67°F. It is a condenser or noncondensable problem. Filter ΔP doubled — service the filter on the OEM curve — but filter ΔP does not raise condensing temperature. Do not let a second fault hide the condenser. Oil supply 138°F: the thermosiphon is either rejecting more compressor heat or the cooler itself is sick. Next comparison is actual 188°F versus the theoretical chart at the new compression ratio (195.8 / 34.3 = 5.71).

Abnormal input: 1.732 × 480 × 358 × 0.86 / 1000. 831.36 × 358 = 297,627; × 0.86 = 255,759; ÷ 1000 = 256.0 kW. About 16 kW extra at the same suction. The exam screen still does not say whether this started this morning or last quarter. In the plant, your log is the timestamp.

Diagnose, then adjust

Once the state points are named, the moves are specific:

• High head, wide wet-bulb approach: wash the evaporative condenser, prove spray and fans, and consider purge if pressure/temperature disagrees with wet bulb.
• High suction superheat: restore evaporator feed (valve, pump, ice, tank level).
• Near-zero suction superheat: protect the screw; chase vessel level and carryover.
• High filter ΔP: change or clean the filter. Low lubrication ΔP is an interlock and a bearing problem, not a filter.
• Slide versus amps mismatch: transmitter, unloading hydraulics, or a VFD speed nobody mentioned.

Exam traps

• Treating NORMAL/ABNORMAL as a stopwatch. Elapsed time is not on the screen.
• Reading psig on a psia table (or the reverse).
• Calling nameplate 300 HP the shaft load.
• Dropping PF when converting amps to kW.
• Plotting the theoretical discharge chart on the wrong pressure scale.
• Memorizing a sample Q&A list instead of computing superheat from this P and T.