Will a Hollow Cone Nozzle Survive a PWM Sprayer? Pulse-Width Modulation Compatibility

Pulse-width modulation (PWM) has become the default way to hold a constant application rate while a sprayer changes speed. Instead of throttling the pump, the controller keeps pressure steady and flips a solenoid open and shut many times a second; the fraction of time it is open, the duty cycle, sets how much liquid actually lands. It is elegant, and it works beautifully with the right nozzle. The question of pwm spray system hollow cone nozzle compatibility is whether a hollow cone pattern can hold its shape while the solenoid is chopping the flow underneath it. The short answer: it can, but only inside a pressure and frequency window you have to respect.

That window is the whole subject of this guide. Get the three numbers right, the pressure floor, the minimum duty cycle, and the pulse frequency against the nozzle’s pattern-refresh time, and a hollow cone runs perfectly under PWM. Get any of the three wrong, and the ring degrades into a ragged dribble that no amount of controller tuning will fix, because the problem is a nozzle pressed outside its window.

What PWM Actually Does to the Liquid

A PWM system does not reduce flow by lowering pressure. It keeps line pressure roughly constant and gates the flow with a fast solenoid. At 80% duty the valve is open most of the time; at 20% it is barely open. The average flow tracks the duty cycle, so the same nozzle delivers a precise rate whether the rig is crawling or flat out.

The math is straightforward: average flow = peak flow at line pressure × duty cycle. A nozzle flowing 10 L/min at full open delivers 8 L/min at 80% duty and 2 L/min at 20% duty, with the pump pressure unchanged. That is the entire trick of PWM: rate control without pressure control, which means the pattern never changes with speed the way a throttled system’s does.

That is the benefit and the catch in one. Because pressure stays up, the nozzle still forms a good pattern during the open part of each pulse. But between pulses the flow stops, and at the start of the next pulse the nozzle has to re-form its pattern from zero. A hollow cone is the pattern most sensitive to that restart, because it depends on a stable swirl to throw liquid into a ring.

Why Hollow Cone Is the Sensitive One

A hollow-cone nozzle forms its donut by spinning the liquid in a swirl chamber or by a tangential entry (spiral nozzles make hollow cone the same way, with the ramp acting as the swirl path). The ring only appears once the swirl is established and the pressure is high enough that the liquid leaves the edge instead of dribbling. Below a minimum pressure the pattern collapses into a weak stream or a ragged spray, and there is no center fill to hide the failure the way a full cone would.

So under PWM, two things have to stay true through every pulse:

• The pressure during the open part of the cycle must clear the nozzle’s minimum pattern pressure.
• The pattern must re-form fast enough that the closed portion of the cycle is a small fraction of the pulse, or you spend too much time spraying nothing.

If either fails, the hollow cone spends part of each cycle not presenting its ring, and the gas or target sees gaps. A full cone hides this: its center fill means even a partially formed pattern still covers the target. The hollow cone has no such grace, which is why compatibility checking matters for hollow cone specifically.

The Pressure Floor Is the First Gate

The single most important number for pwm spray system hollow cone nozzle compatibility is the pressure floor. Pick a hollow cone nozzle whose stable ring forms at or below the pressure your PWM system holds. If your system runs at 3 bar and the nozzle needs 4 bar to make a clean ring, you will never see a good pattern, PWM or not.

The pressure floor is the minimum pressure at which the nozzle produces its full, stable pattern, not the pressure at which it dribbles, and not the catalog’s maximum. Below the floor, a hollow cone collapses; at the floor, it forms a clean ring; above it, the ring tightens and the droplets fine up. Under PWM the valve sees the line pressure during the open part of the cycle, so the floor check is against the line pressure, not the average.

Spiral nozzles help here in dirty service, because their open path forms a pattern at lower pressure than a fine vane swirl chamber, but a hollow-cone spiral still has a minimum, and you must confirm it against the operating pressure, not the peak. Under PWM the valve sees the line pressure; size the nozzle so that pressure is comfortably above its floor across the whole duty range you will use.

Duty Cycle: Do Not Go Too Low

Duty cycle is the second gate. At very low duty, say 10–15%, the valve is closed most of the time. Even if the pattern is perfect during the open slice, the average spray is mostly gap, and any delay in re-forming the ring eats into the already-short open window. Most hollow cone nozzles under PWM stay usable down to roughly 20–30% duty, but below that the pattern quality and the effective rate both suffer.

The practical rule: set your PWM system’s minimum duty to the point where the hollow cone still re-forms cleanly, and let rate changes happen above that floor. If you need very low rates, drop to a smaller nozzle and run it at higher duty rather than choking a large nozzle to a few percent.

Why a smaller nozzle at higher duty is the right answer comes from the arithmetic. A nozzle sized for the maximum rate runs at 15% duty to hit the minimum rate; a nozzle sized for half the maximum runs at 30% duty for the same minimum, with a longer open window and a cleaner ring. The rate range is the same; the pattern quality is not.

Pulse Frequency vs Pattern Refresh

The solenoid flips at a frequency: often tens to low hundreds of hertz depending on the controller. The nozzle has its own pattern-refresh time: how long after flow starts until the ring is stable. If the pulse is shorter than the refresh time, the nozzle never finishes forming the pattern before the valve closes, and you get a weak, variable spray every cycle.

Compatibility, then, is a frequency match. The constraint is simple: the open time of the shortest pulse you use must exceed the nozzle’s refresh time. Open time = duty ÷ frequency, so at 20 Hz a 30% duty pulse is open for 15 ms; at 60% duty it is open for 30 ms. A nozzle that refreshes in 5 ms is fine in both; one that takes 20 ms only works in the second case.

Solenoid frequency	Duty cycle	Pulse open time	Nozzle refresh 5 ms	Nozzle refresh 20 ms
10 Hz	20%	20 ms	Works: 15 ms of stable ring	Marginal: ring never settles
10 Hz	50%	50 ms	Works	Works
20 Hz	30%	15 ms	Works: 10 ms of stable ring	Fails: ring never forms
30 Hz	20%	6.7 ms	Marginal	Fails
30 Hz	60%	20 ms	Works	Marginal

Pulse open time = duty ÷ frequency. The usable spray window is open time minus refresh time.

Two consequences follow from the table. First, higher frequency is not automatically better: at the same duty it means a shorter open window, which is harder on the nozzle, not easier. Second, low duty and high frequency together are the killing combination: short open windows at high repetition rates, the worst case for a hollow cone. If your controller runs a high frequency, confirm the nozzle’s refresh is shorter than the open portion of the shortest pulse you will use. Vendors rarely print refresh time, so the field check is simple: watch the pattern at your minimum duty and lowest speed: if it breaks up, the frequency or the duty is outside the window.

Solenoid and Line Effects You Can Hear

Two real-world issues show up once a system is running. First, if the solenoid and nozzle are far apart on a long line, the pressure pulse softens by the time it reaches the nozzle, which can actually help pattern stability but hurts rate precision: keep the valve close to the nozzle. Second, a worn solenoid that does not fully close lets a dribble through during the “off” part, which blurs the duty cycle and shows up as drift in the applied rate. Neither is a nozzle fault, but both get blamed on the hollow cone.

A third, quieter one: the line itself becomes an accumulator. A long, compliant hose between valve and nozzle stretches under each pulse and relaxes during the closed part, rounding off the flow square wave into a gentler ramp. That softens the pattern (good) and delays the peak flow (bad for precision at high frequencies). The fix is short, stiff line between valve and nozzle, and the controller’s frequency set with the installed line length in mind.

Droplet Consistency Under PWM

PWM’s second effect on a hollow cone is on droplet size, and it is worth separating from pattern shape. In a pressure nozzle, droplet size is set mostly by pressure, not by flow: as pressure rises, droplets fine down; as it falls, they coarsen. Because PWM holds line pressure constant, the nozzle produces essentially the same droplet spectrum during every open pulse. That is the good news, and it is why PWM is so much better for rate control than throttling, which coarsens the spray as the pressure drops.

The bad news is the start of every pulse. When the solenoid opens, the nozzle spends the first few milliseconds building pressure and swirl, and in that transient it throws larger, faster drops than its steady-state spectrum. A nozzle whose ring refreshes in 5 ms spends 5 ms of every pulse in that coarse transient; one that needs 20 ms spends 20 ms there. The applied droplet distribution is therefore a time-weighted mix: the steady-state spectrum during the stable part of the pulse, plus a coarse fraction from every start-up.

The consequence is that droplet consistency under PWM is really the same refresh-time question wearing a different hat:

Duty cycle	Stable spray share (5 ms refresh, 20 Hz)	Droplet character
80%	88% of open time stable	Near steady-state spectrum
50%	80% of open time stable	Slight coarse fraction
30%	67% of open time stable	Noticeable coarse fraction
20%	50% of open time stable	Coarse fraction material

Stable share = (open time − refresh) ÷ open time. A slower-refreshing nozzle moves every row to the left.

Two practical rules follow. First, for duties where droplet size is itself the spec, drift control, chemical application, coverage-quality targets, keep duty up and nozzle size down, so the stable share of every pulse stays high. Second, lower frequency at the same duty gives a longer open window, which shrinks the coarse start-up fraction as a share of the pulse. If your controller allows it and the pattern holds, running at 10–15 Hz instead of 20–30 Hz buys measurably more consistent droplets from the same nozzle.

Spiral Nozzles as the Hollow-Cone Option Under PWM

When the service is also dirty or recirculated, a spiral nozzle giving a hollow-cone pattern is often the safest PWM choice: the open path resists clogging, it forms a ring at lower pressure than a vane swirl, and it refreshes quickly. The open ramp reaches full swirl faster than a multi-chamber vane design. The trade is the one spiral nozzles always carry: a wide, not tightly uniform, droplet distribution. For absorption and gas-contact duties under PWM that is usually fine; for a duty that needs a precise droplet band it is not, and you would pick a vane hollow cone and accept tighter pressure and clogging limits.

Spiral nozzles and spiral jet variants both sit in this family; the jet form trades the ring for a more directed stream when the duty is a focused wash rather than area coverage. Matching the form to the PWM window is the same exercise: confirm pressure floor, minimum duty and refresh.

The spiral nozzle product range lists angle, flow and the low-pressure band per pattern, which is what you need to confirm the floor before you commit a controller to a duty range.

Sizing the Nozzle for the Rate Range

PWM changes how you size a nozzle, because the rate range, not the maximum rate alone, becomes the spec. The sequence:

1. Define the rate range as a ratio: maximum rate ÷ minimum rate. A system that must go from 100% down to 20% has a 5:1 range.
2. Pick the smallest nozzle that hits the maximum rate at the system pressure, with headroom. Flow scales with the square root of pressure, so flow at pressure P = rated flow × √(P ÷ rated pressure).
3. Check the minimum rate against the duty floor. If the smallest workable nozzle still needs a duty below 20–30% to reach the minimum rate, the range is too wide for one nozzle: split it across two sizes and switch between them, or accept the pattern loss at the low end.
4. Confirm the pressure floor and refresh time against the shortest pulse, using the table above.
5. Field-verify at the worst case: lowest speed, lowest duty, highest frequency: the pattern should still read as a ring.

The worked example: the system holds 4 bar and needs 4–20 L/min. A nozzle rated 20 L/min at 4 bar hits the maximum at 100% duty. The minimum rate needs 20% duty: inside the floor for most hollow cones, but check the ring at that duty before committing. A nozzle rated 10 L/min at 4 bar would hit the maximum only at 200% duty, impossible, so the 20 L/min body is the right size, and the duty floor is the constraint to verify.

A Pre-Install Compatibility Check

1. Note the line pressure your PWM system holds, and the solenoid frequency.
2. Pick a hollow cone nozzle whose stable ring forms below that pressure.
3. Set the controller’s minimum duty above the point where the ring re-forms.
4. Confirm pulse-open time exceeds the nozzle’s pattern refresh at that duty: open time = duty ÷ frequency.
5. Keep the solenoid close to the nozzle on a short, stiff line.
6. Field-check the pattern at lowest speed and lowest duty; if it breaks, raise duty or drop nozzle size.

Troubleshooting a PWM Hollow Cone

Symptom	Likely cause	Check	Fix
Ring breaks up at low speed	Duty below the pattern floor	Pattern at minimum duty	Raise minimum duty; smaller nozzle
Dribble between pulses	Worn solenoid not fully closing	Off-period flow	Replace solenoid
Rate drifts from setpoint	Soft line rounding the pulse	Line length and stiffness	Shorten line; stiffer hose
Ring never forms at any duty	Pressure below the floor	Pressure at the nozzle	Raise line pressure; bigger floor margin
Pattern worse at high frequency	Open time below refresh time	Pulse timing (duty ÷ frequency)	Lower frequency or duty up
Clogging on dirty water	Vane chamber bridging	Free passage vs solids	Spiral hollow cone

Frequently Asked Questions

Can I run a hollow cone nozzle on a PWM system? Yes, inside a window: line pressure above the nozzle’s minimum pattern pressure, duty cycle above the re-form floor (roughly 20–30% for most bodies), and pulse open time longer than the pattern refresh time.

What is the minimum duty cycle for a hollow cone under PWM? Typically 20–30% before the pattern quality and effective rate both suffer. Below that, drop to a smaller nozzle and run it at higher duty instead of choking a large one.

Does higher pulse frequency give better control? Not necessarily for the pattern. Higher frequency means a shorter open window at the same duty: harder on the nozzle. What matters is that open time (duty ÷ frequency) exceeds the nozzle’s refresh time.

Why does my hollow cone dribble between pulses? Most likely a worn solenoid that does not fully close, or a long soft line that rounds off the pulse. Check the off-period flow before blaming the nozzle.

Are spiral hollow cones better under PWM? For dirty service, yes: the open ramp forms a ring at lower pressure and refreshes faster than a vane swirl, and it resists clogging. The trade is a wider droplet distribution.

How do I choose the nozzle size for a PWM range? Size the smallest body that hits your maximum rate at system pressure, then check the minimum rate against the duty floor. If the range needs duty below the floor, split it across two nozzle sizes.

What pressure should my PWM system hold for a hollow cone? Whatever clears the nozzle’s minimum pattern pressure with margin. Confirm against the operating pressure, not the peak. The valve sees the line pressure during each open pulse.

Why does the pattern break only at low speed? Low speed means low duty, which means short open windows. The ring has less time to re-form. Raise the minimum duty, lower the frequency, or use a smaller nozzle at higher duty.

If you are unsure where your PWM system’s window actually sits, send the controller model, line pressure and the rate range you need. The engineering desk will match a hollow-cone nozzle, spiral or vane, to the pressure floor and duty cycle so the pattern holds through every pulse. For the broader question of why spray patterns drift from spec in the first place, the pattern drift guide covers the field side, and the dirty-water spiral guide covers keeping the ring alive when the feed is not clean.