A tank-mounted duplex air compressor combines two independently driven pump assemblies with a shared receiver. An alternating lead/lag control can share running time between the units and call for both when demand requires. This calculator compares average demand with two equal delivered capacities.
Duplex Air Compressor Interactive Calculator
Compare average air demand with the delivered capacity of two equal pumps. Watch the independent assemblies alternate lead and lag roles on their shared receiver.
Equation Used
- Two equal delivered capacities at the same working condition.
- Constant average demand and constant running delivery.
- On/off scheduling; losses included in your inputs or omitted.
- Receiver pressure, switching intervals and thermal limits are not calculated.
Use the same air-volume reference condition for demand and delivered capacity. Run fractions do not establish allowable duty.
Two pump assemblies share one receiver
The drawing shows two electric motors, belt drives and finned compressor pumps mounted above a horizontal air receiver. Separate discharge connections and check valves lead into the common receiver. An alternator assigns the lead role.
The animation alternates that role between assemblies A and B in two equal illustrative slots. Within each slot, the coloured running indicator and rotating belt pulleys distinguish a running unit from a stopped unit. Delivery markers appear on its discharge pipe only while that unit runs.
The fractions of time shown agree with the simplified capacity budget below. The slot duration is a display choice. Real pressure-switch cycling depends on receiver storage, pressure settings and changing consumption; none of those is inferred from this two-input calculator.
Compare demand at a common reference condition
Enter average demand Qd and delivered capacity Qp for one pump. Both must use the same air-volume reference convention and working condition. Do not mix free-air delivery with compressed line volume without conversion.
The tool assumes the two pumps have equal capacity. The illustration does not determine that capacity from the pictured cylinders or pulley size. It also does not translate motor horsepower into delivery.
If average demand exceeds two-pump capacity, the result is a negative capacity margin. The receiver cannot eliminate that sustained shortfall. If one pump is unavailable, compare demand against the remaining single-pump capacity rather than assuming the package retains full redundancy.
A simplified lead/lag flow budget
Let r=Qd/Qp. The lead-role run fraction is L=min(r,1). The lag-role run fraction is G=min(max(r−1,0),1). Multiply either fraction by 60 to obtain equivalent running minutes per hour for that role.
When Qd≤2Qp, the average supply is Qp(L+G)=Qd. Above this capacity, both fractions stop at 1 and the unmet average demand is Qd−2Qp. The signed capacity margin shown is 2Qp−Qd.
With equal alternation of the two roles, each physical assembly runs for 30(L+G) minutes per hour on average. This distinguishes a role-based duty calculation from the running time of motor A or B.
The model assumes on/off operation at constant delivered capacity and neglects startup losses, unloading consumption and leakage beyond any amount included in Qd. It is a flow allocation, not a pressure or thermal model.
Three demand levels
At the default 7.5 CFM demand and 25 CFM per pump, the lead role runs 30% of the time, or 18 min/hr. The lag role is not needed. With equal alternation, each assembly averages 9 min/hr. The full-capacity margin is 42.5 CFM.
At 37.5 CFM demand with the same pumps, the lead role requires 100% and the lag role 50%. Each physical assembly averages 45 min/hr after equal alternation. The margin is 12.5 CFM.
At 60 CFM demand, both 25 CFM units run continuously in the idealized budget, but supply only 50 CFM. The −10 CFM margin exposes the shortfall. These fractions do not establish whether continuous operation is within the pump manufacturer’s allowed duty.
What the budget does not size
The receiver stores air, but its volume and pressure range are not inputs here. The page therefore does not calculate time to fill, pressure drop, starts per hour or a pressure-switch setting. Receiver pressure is outside this flow-budget model.
Pulley proportions, pump casings, guards, check valves and discharge routing are illustrative. Internal valve timing, compression temperature, belt loading and component selection are outside this model.
Two units can improve availability only within the remaining capacity when one is stopped. No blanket service-life, motor-size, duty-limit or redundancy guarantee follows from having two pumps.
Duplex compressor calculator questions
Does lead always mean assembly A?
No. The animation swaps the lead and lag roles between A and B. The output labels describe roles, and the drawing also reports each unit’s average running time after equal alternation.
Why is the margin negative?
Average demand exceeds the sum of the two entered capacities. Capping this result at zero would hide the size of the deficit.
Why are the motors sometimes stopped while demand is nonzero?
The run fractions allocate average delivery over the illustrative schedule. A receiver buffers real cycling, but this tool does not solve its pressure.
Does 100% mean continuous operation is permitted?
No. It means the ideal budget requires continuous running. The equipment’s permissible duty must come from its own specification.
What happens at zero demand?
Both calculated run fractions become zero and both assemblies stop. The total unused capacity remains 2Qp.
Reference and model basis
Powerex Climate Control Duplex manual, pages 1–2, provides the physical package arrangement and describes alternating lead/lag operation. The drawing shows those component roles with schematic dimensions.
The equal-capacity equations and illustrative schedule above are an independent flow-budget model. They do not reproduce the manual’s pressure-switch settings or certify a particular installation.
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