Tank vs Tankless Air Compressor: How to Choose a Small Compressor
HCEM Engineering TeamShare
A tank (receiver) compressor stores compressed air, so it can cover short peaks above the pump's own output, let the motor stop between cycles, smooth the piston's pulses and collect condensate. A tankless compressor delivers air straight from the pump, so it must supply the full demand at every moment and runs whenever air is used. Neither is better in general. Tankless suits a steady, small flow inside a machine; a tank suits demand that comes in bursts.
This guide explains what the receiver really does, how to size one with a simple formula, how tank size changes motor starts, and when a tankless pump is the right choice for an equipment design. The examples use datasheet values from our own oil-free piston compressors.
What is the difference between a tank and a tankless air compressor?
Both use the same pump. The difference is what sits between the pump and the point of use:
- Tank (receiver-mounted). The pump fills a pressure vessel. A pressure switch starts the motor at a lower pressure (cut-in) and stops it at a higher pressure (cut-out). The application draws from the stored air.
- Tankless (direct, on-demand). The pump discharges straight into the line or the device. There is no stored volume beyond the tubing, so the pump runs for as long as air is needed, and its flow at the working pressure must be at least the demand at every moment.
The deciding question is the shape of your demand. If the flow you need is steady and below what the pump delivers at your pressure, a tank adds weight and little else. If the demand comes in peaks above the pump's output, or is so low that the pump would run almost all the time for very little air, a receiver pays for itself.
What does the air tank on a compressor actually do?
A receiver does four separate jobs. It is worth knowing which one you actually need, because a small buffer volume can do some of them without a full tank.
- Buffers peak demand. Between cut-out and cut-in the tank releases stored air, so for a short time the application can take more than the pump delivers. This only works for peaks: if the average demand is higher than the pump's flow, no tank size will keep up.
- Lets the motor stop. With storage, the motor runs to refill the tank and then rests. Without it, the motor must run whenever air is drawn, or start and stop with every small demand.
- Smooths pulsation. A piston pump delivers air in pulses, one per compression stroke. The tank's volume damps those pulses, so pressure at the outlet is steadier. A regulator after the tank holds the outlet at a set pressure while the tank swings between cut-in and cut-out.
- Collects condensate. Air cools in the tank, and some of the water vapour it carries condenses and settles at the bottom, where it can be drained instead of travelling into your device.
For a closer look at pulsation and how to damp it without a full receiver, see our guide to pump pulsation and airflow stability.
How big should an air receiver be?
For a first estimate, size the receiver to cover the peak with the pressure band you can accept:
V = t × (C − S) × patm ÷ (p1 − p2)
| Symbol | Meaning | Unit (metric) | Unit (imperial) |
|---|---|---|---|
| V | Receiver volume | litres | ft³ |
| t | Duration of the peak | minutes | minutes |
| C | Demand during the peak, as free air | L/min | cfm |
| S | Pump delivery during the peak, as free air at the working pressure | L/min | cfm |
| patm | Atmospheric pressure | 1.013 bar | 14.7 psi |
| p1 − p2 | Pressure the tank may fall during the peak (start minus minimum) | bar | psi |
Three points keep the units honest. C and S must both be free air flows (air measured at atmospheric conditions), which is how compressor flow is normally quoted; if you need a refresher, see what CFM means. The pressure difference is the same whether you write both pressures as gauge or as absolute, but do not mix the two. And p2 is the minimum the tank may reach, which is your point-of-use pressure plus the losses in the regulator, filters and tubing.
Worked example. A device needs 80 L/min of free air for 15 seconds at a time. The pump delivers 40 L/min at the working pressure. The tank may fall from 7 bar to 5 bar gauge during the peak.
- t = 15 s = 0.25 min
- C − S = 80 − 40 = 40 L/min
- p1 − p2 = 7 − 5 = 2 bar
- V = 0.25 × 40 × 1.013 ÷ 2 ≈ 5.1 litres
So about 5 litres of receiver is the minimum, and a 6 litre tank leaves a small margin. Then check recovery: a 6 litre tank holds 6 × 2 ÷ 1.013 ≈ 11.8 litres of free air between 7 and 5 bar, and a 40 L/min pump refills that in about 18 seconds with no other demand. If the next peak comes sooner than that, you need a bigger tank, a wider pressure band or a larger pump.
The formula assumes the pump is already running throughout the peak and that the air stays at constant temperature. In practice the pump's delivery changes with pressure, freshly compressed air is warm and shrinks as it cools, and every hose and fitting adds loss, so add margin and confirm on the real system. Convert units with our CFM to L/min converter, estimate actuator demand with the pneumatic cylinder air consumption calculator, and for a full worksheet with several demand events see receiver sizing for intermittent OEM demand.
How does tank size affect motor starts and duty cycle?
Every start puts inrush current through the motor windings and heats them, and a compressor motor is only rated for a certain number of starts per hour. The receiver volume and the pressure band set how often the motor starts.
The worst case is when the average demand is exactly half the pump's delivery: the fill time and the drain time are then equal, and the cycle is at its shortest. The shortest cycle is:
Tmin = 4 × V × (p1 − p2) ÷ (patm × S)
| Receiver | Pressure band | Pump delivery | Shortest cycle | Most starts per hour |
|---|---|---|---|---|
| 6 L | 7 to 5 bar | 40 L/min | about 1.2 min | about 51 |
| 30 L | 7 to 5 bar | 40 L/min | about 5.9 min | about 10 |
Five times the volume gives one fifth of the starts. Widening the pressure band works the same way, at the cost of a bigger pressure swing in the tank. Compare the result with the starts per hour the motor and pump are rated for, and check the rated duty cycle at your working pressure. Many small tank compressors also use an unloader valve on the pressure switch that vents the pump head when the motor stops, so the next start is not against full pressure. Our duty cycle guide covers continuous and intermittent ratings in more detail.
When does a tankless compressor make sense?
Tankless is the natural choice when the demand is continuous, steady and within the pump's continuous rating. In that case a receiver would only fill once and then sit at pressure while the pump runs anyway. Typical cases inside OEM equipment:
- Oxygen concentrators. The compressor feeds the sieve beds of a pressure swing adsorption (PSA) system at low pressure (our oxygen concentrator pumps are rated at 140 to 200 kPa) all the time the device is on. The beds and tubing already provide some volume, so a separate tank is rarely needed.
- Analysers and sampling devices that need a small, constant purge or carrier flow.
- Ozone generators, humidity generators and similar process equipment fed with a steady stream of clean air.
- Aeration of ponds and tanks, where the pump runs continuously at very low pressure.
In these designs the pump is a component, so the design work moves elsewhere: choose a pump rated for continuous duty at your pressure and ambient temperature, give it cooling air (see our bare pump integration guide), fit a pressure relief valve in case the outlet is blocked, and add a small buffer volume or damper if a sensor or flow meter downstream is sensitive to pulsation.
A tankless setup is the wrong choice when demand comes in bursts larger than the pump's flow, when air is needed only now and then (the motor would start for every small draw), or when the application needs stored air ready the moment it is switched on.
Tank vs tankless: how do they compare?
| Point | With receiver | Tankless (direct) |
|---|---|---|
| Peak demand above pump flow | Covered for a time set by tank volume and pressure band | Not possible: pump flow must meet demand at every moment |
| Motor running | Runs to refill, then stops | Runs whenever air is drawn |
| Pressure at the outlet | Steady after a regulator | Pulsating unless a buffer or damper is added |
| Condensate | Collects in the tank and must be drained | Travels downstream and condenses where the air cools |
| Size and weight | Larger and heavier | Compact, easy to build into a machine |
| Pressure vessel duties | The vessel needs a safety valve, a drain and, depending on size and pressure, documented examination | No vessel, but a relief valve is still good practice |
| Best fit | Intermittent tools and actuators, dental and laboratory air, peaks above pump output | Continuous small flow inside a device: oxygen concentrators, analysers, aeration |
Which HCEM compressors come with or without a tank?
Here are datasheet values for some of our oil-free piston compressors, with and without a receiver. Where a listing covers two models or voltages, both values are shown.
| Model | Receiver | Flow at pressure | Power | Noise | Weight |
|---|---|---|---|---|---|
| HC100 Medical Oxygenation Pump | No tank | ≥30 L/min, or ≥38 L/min, at 200 kPa | ≤180 W, or ≤220 W | ≤52 dB(A), or ≤55 dB(A) | 3.4 kg |
| HC210 Medical Oxygenation Pump | No tank | ≥50 L/min at 140 kPa | ≤230 W | ≤52 dB(A) | 4.3 kg |
| HC280 Oilless Air Pump | No tank | 20 L/min, or 25 L/min, at 700 kPa | ≤340 W, or ≤370 W | ≤63 dB(A), or ≤67 dB(A) | 6.8 kg |
| HC580 Oilless Air Pump | No tank | 60 L/min at 700 kPa | ≤690 W | ≤65 dB(A) | 10.1 kg |
| HC1100 Oilless Air Pump | No tank | 100 L/min at 800 kPa | ≤1100 W | ≤70 dB(A) | 17.2 kg |
| HC5501-6 / HC7501-6 | 6 L | 40 L/min, or 60 L/min, at 700 kPa | 470 W, or 680 W | see datasheet | 18 kg, or 18.5 kg |
| HC1530X | 30 L | 40 L/min, or 60 L/min, at 8 bar | 470 W, or 680 W | ≤56 dB(A) | 62 kg, or 65 kg |
The table shows the trade-off plainly. The bare pumps weigh 3.4 to 17.2 kg; the same 40 to 60 L/min with a receiver weighs 18 kg on a 6 litre tank and over 60 kg on a 30 litre one. A 6 litre receiver, as on the HC5501-6, is enough for the worked example above; the 30 litre HC1530X cuts motor starts by a factor of five at the same flow. Browse the oil-free compressors with receivers, the AC and DC oil-free air pumps for building into a machine, and our oxygen concentrator compressors.
Is a tankless air compressor quieter?
Not by itself. The noise comes from the pump and motor, not from the tank. What changes is when you hear it. A tank compressor is silent between cycles but you notice every start; a tankless pump runs at a steady level for as long as air is drawn. For a continuous low-pressure flow, a small pump running steadily can be very quiet: the HC100 and HC210 above are rated at ≤52 dB(A). For intermittent use, a receiver means the motor runs only part of the time. Compare dB(A) values at the same measuring distance, and see our guide to reducing compressor noise with enclosures and isolation.
What happens to condensate with and without a tank?
Oil-free does not mean water-free. Every compressor takes in water vapour with the air, and compression concentrates it. As a rough example: air at 20 °C and 60% relative humidity carries about 10 g of water per m³. Compressed to 7 bar gauge (8 bar absolute) and cooled back to 20 °C, the same air takes one eighth of the volume and can hold only about 2 g as vapour. The other 8 g or so per m³ of free air becomes liquid. A pump delivering 40 L/min for an hour moves 2.4 m³ of free air, so about 20 g (20 mL) of water condenses somewhere downstream, more on a warm, humid day.
With a tank, much of that water collects at the bottom of the vessel. Drain it at least daily, or fit an automatic drain: standing water corrodes the tank and takes up storage volume. Without a tank, the water stays in the hot air until it cools in the tubing or inside your device. If the application cannot tolerate liquid water, add a water separator with a drain, or a dryer. Our guides to dew point and air dryers and ISO 8573-1 air purity classes explain how to specify dry, clean air.
Common questions
Can a tankless air compressor run continuously?
Only if the pump is rated for continuous duty at your working pressure and ambient temperature, and has enough cooling air. A pump rated for intermittent duty will overheat if it runs without a break, tank or no tank.
Can I add a tank to a tankless compressor?
Yes. A separate receiver can be added after the pump, but it then needs its own safety valve, drain and pressure switch, and the pump needs a check valve so the tank does not bleed back through it. Size the tank with the formula above.
Does a bigger tank give more pressure or more air?
Neither. The maximum pressure is set by the pump and the pressure switch, and the continuous flow is set by the pump. A bigger tank only stores more air, so it covers longer peaks and makes the motor start less often.
Is a small compressor tank a pressure vessel under UK rules?
Yes. In Great Britain, compressed air systems fall under the Pressure Systems Safety Regulations 2000, and HSE guidance links the need for a written scheme of examination to the pressure × volume of the largest vessel (250 bar·litres). A 6 litre tank at 8 bar is 48 bar·litres; a 50 litre tank at 8 bar is 400 bar·litres. Check the current HSE guidance for your own case.
How often should I drain the tank?
At least once a day in normal use, and more often in warm, humid conditions or with long running times. An automatic drain removes the need to remember.
For more on comparing small compressors, see our small air compressor buying guide and our dental compressor guide, a typical receiver application. Or send us your flow, working pressure, supply voltage and duty cycle, and whether your demand is steady or comes in peaks, and our engineers will suggest a pump, with or without a receiver, including OEM versions for your equipment.