Compressor Short-Cycle Protection
Estimated cycles per hour from the ASHRAE/AHRI part-load cycling parabola, on/off time.
Example
You enter
- System type single
- Load fraction (% of design) 50
You get
- Cph estimated 6
- On time (min) 5
- Off time (min) 5
- Minimum runtime 10
Checked against published worked examples from: Copeland / ASHRAE.
Details, formula, and sources
ScopeEstimated cycles per hour from the ASHRAE/AHRI part-load cycling parabola, on/off time, and the minimum oil-return runtime and pressure-equalization delay by system type, flagging oversized single-stage short-cycling. Per Copeland AE Bulletin 17-1226.
Formula or tableCycle rate N = N_max × 4 × X × (1 − X), where X is the runtime (load) fraction and N_max is the per-hour ceiling; the parabola peaks at X = 0.5. On-time = X × 60 / N; off-time = (1 − X) × 60 / N. Short-cycling is flagged when the estimated on-time is below the minimum oil-return runtime or the observed cycles exceed the ceiling.
Source and editionCopeland Application Engineering Bulletin 17-1226 by name; ASHRAE Fundamentals 2021. The ASHRAE/AHRI part-load cycling model by name.
Free accessCopeland AE bulletins free at copeland.com/literature; ASHRAE Handbook TOC free at ashrae.org.
What governsEstimate. AHJ and licensed mechanical contractor govern. ACCA Manual J / D / S supersede rules of thumb.
API field namessystem_type, load_fraction_pct, cph_estimated, on_time_min, off_time_min, min_runtime_min
- Cycling parabola N = N_max × 4 × X × (1 − X)ASHRAE/AHRI part-load cycling model
- Single-stage minimum runtime 10 min on, 5 min offCopeland AE Bulletin 17-1226 / industry rule of thumb
- Cycles-per-hour ceiling 6 single-stage, 8 two-stagecompressor-protection guidance