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1. Purpose and Engineering Decision
A load/unload compressor can consume substantial power while producing little or no useful air during the unloaded portion of its cycle.
The calculator does not assume that unloaded power is zero. The user enters measured or verified loaded and unloaded package kW, then the cycle times or fractions. The result remains traceable to those inputs.
Mode | Primary output |
|---|---|
Cycle timing mode | Average kW from load / unload durations |
Fraction mode | Average kW from percent loaded and unloaded |
Annual energy mode | kWh/y from average kW and operating hours |
Scenario comparison | Energy and cost delta before vs after a control/storage change |
Air-normalized mode | Average kW divided by delivered flow over the same observation period |
2. Core Cycle Equations
Cycle Time = t_load + t_unload
Loaded Fraction = t_load / Cycle Time
Unloaded Fraction = t_unload / Cycle Time
Average kW = (P_load x t_load + P_unload x t_unload) / Cycle Time
If additional states are measured, such as off-load blowdown, idle, stopped or variable-speed trim, the engine can extend the weighted-average equation to any number of mutually exclusive states.
Average kW = Sum(P_i x t_i) / Sum(t_i)
3. Annual Energy and Cost
Annual kWh = Average kW x Annual Operating Hours
Annual Cost = Annual kWh x Electricity Tariff
The annual operating-hours input must represent the hours in which the defined cycle pattern actually occurs. If the machine operates under several distinct demand patterns, each pattern should be calculated separately and then aggregated.
4. Unload Energy Penalty
A useful diagnostic is the energy consumed during unloaded time. This is not automatically 'waste' in a strict thermodynamic sense because some unload energy may be inherent to the machine's control method, but it is the energy component available for engineering scrutiny.
Unload kWh = P_unload x Unload Fraction x Annual Operating Hours
Unload Energy Share (%) = Unload kWh / Annual kWh x 100
5. Delivered-Air Denominator
When a user wants cycle-specific power, the delivered-air denominator must cover the same observation interval as the power data. If the machine delivers rated flow while loaded and approximately zero flow while unloaded, the cycle-average delivered flow may be estimated from loaded flow multiplied by loaded fraction. Measured flow is preferred whenever available.
Q_avg = Q_loaded x Loaded Fraction [simplified load/unload estimate]
Cycle Specific Power = Average kW / Q_avg
If the compressor continues delivering measurable flow in an unloaded or modulating state, that flow must be entered rather than assumed to be zero.
6. Input Model
Input group | Field | Validation / note |
|---|---|---|
Power | Loaded package kW | Must be > 0 |
Power | Unloaded package kW | Must be >= 0 |
Timing | Loaded duration | > 0 in timing mode |
Timing | Unloaded duration | >= 0 |
Timing | Cycle count / hour | Optional cross-check |
Flow | Loaded delivered flow | Optional unless specific-power mode is used |
Flow | Unloaded delivered flow | Optional; default zero only when explicitly selected |
Annualization | Operating hours / year | >= 0 |
Economics | Electricity tariff | Optional; >= 0 |
Scenario | Before / after label | Used for comparison |
Metadata | Measured / controller / estimated | Confidence classification |
7. Cycle Frequency Cross-Check
Cycles per Hour = 3600 / Cycle Time (seconds)
A large mismatch indicates that the timing sample is not representative or the observation window contains additional states.
8. Reference Test Case A - Short Cycling Compressor
A compressor draws 110 kW loaded and 38 kW unloaded. A representative cycle is 45 seconds loaded and 30 seconds unloaded. The machine operates 6,000 hours per year.
Calculated item | Result |
|---|---|
Cycle time | 75 s |
Loaded fraction | 60.0% |
Unloaded fraction | 40.0% |
Cycles per hour | 48.0 |
Average power | 81.2 kW |
Annual energy | 487,200 kWh/y |
Unload-energy exposure | 91,200 kWh/y (18.7% of total) |
9. Reference Test Case B - Storage / Control Improvement
Assume a change in storage or control increases loaded time to 90 seconds and unload time to 30 seconds while loaded and unloaded kW remain 110 and 38 kW. This does not prove the change is optimal, but it shows how longer productive cycles alter weighted power.
Scenario | Average kW | Annual kWh |
|---|---|---|
Before: 45 s load / 30 s unload | 81.2 | 487,200 |
After: 90 s load / 30 s unload | 92.0 | 552,000 |
Difference | -10.8 kW | -64,800 kWh/y |
Because average kW can rise when loaded fraction rises, the engineering decision must also consider useful air delivered.
10. Reference Test Case C - Cycle Specific Power
Using Test Case A, assume the compressor delivers 20 m3/min FAD while loaded and negligible air while unloaded. The cycle-average flow is therefore 20 x loaded fraction.
Scenario | Average flow | Average kW | Cycle specific power |
|---|---|---|---|
Before | 12.00 m3/min | 81.2 | 6.77 kW/(m3/min) |
After | 15.00 m3/min | 92.0 | 6.13 kW/(m3/min) |
The example shows why useful-air normalization matters. A scenario with higher average kW can still use less energy per unit of delivered air if the unloaded fraction is reduced substantially.
11. Stop / Auto-Stop State
Many compressors can stop after remaining unloaded for a defined delay.
Average kW = (P_load t_load + P_unload t_unload + P_stop t_stop) / Total Time
The calculator must not assume instantaneous restart or ignore restart limits. Mechanical starts per hour, motor thermal constraints and control logic are separate engineering checks.
12. Cycle Timing and Receiver Interaction
Receiver volume and control pressure band influence how quickly system pressure rises and falls, which in turn affects load/unload timing.
13. Confidence Model
Input source | Suggested confidence |
|---|---|
Synchronized power logger + controller state + pressure trend | High |
Measured kW + reliable controller load/unload timers | High |
Controller hours / cycle counters over long interval | Medium-High |
Short manual stopwatch sample | Medium / preliminary |
Nameplate kW + assumed unload percentage | Low / preliminary |
14. Validation and Guardrails
Condition | Required behaviour |
|---|---|
Loaded kW <= 0 | Block calculation |
Unloaded kW < 0 | Block calculation |
Cycle time <= 0 | Block timing mode |
Loaded + unloaded fractions != 100% | Normalize only if user explicitly accepts; otherwise warn |
Cycles/hour inconsistent with entered timing | Show mismatch warning |
Annual hours < 0 | Block annualization |
Tariff < 0 | Block cost calculation |
Average delivered flow <= 0 in specific-power mode | Block SP result |
Power and cycle-state timestamps not aligned | Lower confidence |
Very high cycles/hour | Flag for mechanical/control review; do not prescribe a universal limit |
15. Output Specification
Output | Display rule |
|---|---|
Loaded fraction | % |
Unloaded fraction | % |
Cycles per hour | From measured cycle time |
Average package kW | Weighted result |
Annual kWh | From entered annual hours |
Unload-energy exposure | kWh/y and % of total |
Average delivered flow | When flow inputs exist |
Cycle specific power | When synchronized flow basis exists |
Scenario delta | Average kW, kWh, cost and SP difference |
Confidence / warnings | Always visible |