Howden Roots Blower vs. Howden Screw Compressor: A Plant Buyer's Perspective

Choose a Howden roots blower for low-pressure, high-volume gas movement, and choose a Howden screw compressor for higher-pressure compression. That is the shortest useful answer I can give after managing capital equipment purchases for a specialty chemical plant for the past five years.

I'm not an engineer. I'm the in-house buyer who took over purchasing in 2020, manages roughly $1M a year across 60-80 orders, and reports to both operations and finance. I've selected, spec'd, and lived through enough compressor commissioning headaches to know what matters when you're putting together a capital request.

The difference that actually matters

A roots blower moves gas by trapping it between rotating lobes and pushing it through the discharge. The flow is continuous, but the pressure capability is limited. A Howden roots blower will give you high volume at perhaps 0.3 to 1 bar gauge, which is enough for aeration, pneumatic conveying, or gas boosting in some processes.

A screw compressor compresses gas continuously via meshing rotors. It can reach much higher pressures, and it is often the better choice when your process needs dependable, continuous compressed gas. In practice, if you need more than about 1.5 bar, I stop looking at blowers and start looking at screw compressors.

I ask every vendor to state flow according to ISO 1217. That standard gives a common reference for inlet conditions, and it makes quotes comparable. A vendor who can't produce that data usually can't produce a reliable invoice either.

When a Howden roots blower is the right call

In 2023, we replaced a 15-year-old blower on a wastewater aeration basin. The quote we accepted was for a Howden roots blower package. It wasn't the cheapest option, but the price included a performance test, a clear motor duty, and a delivery date the contractor actually kept. These blowers move a ton of air at a modest pressure, and they're simple enough for our maintenance team to troubleshoot.

That's where I learned my rookie mistake. In my first year, I assumed a roots blower with a bigger motor would automatically push more pressure. It doesn't work that way. The lobe profile and inlet size determine flow; horsepower only lets the motor survive the load. I nearly signed off on a higher-horsepower motor that would not have made the blower reach the process pressure. The vendor caught it before build, and I learned to read the data sheet before pushing a purchase order through.

When a Howden screw compressor is the better buy

If your process requires gas at 2 bar or higher, a Howden screw compressor is usually the better machine. It is smoother than a reciprocating compressor, takes up less space for the capacity, and can run continuously. Plant air, refrigeration, and gas transfer are the first applications I think of.

We installed a Howden screw compressor for an ammonia refrigeration loop in 2024. I say 'we' because I mostly watched the engineers. My part was making sure the quoted capacity included the actual summer operating conditions, not the ideal data sheet number. If your process can't tolerate oil carryover, ask for oil-free screw technology before you get to price. It changes the package and the budget enough to deserve its own line item.

The communication failure that still bothers me: I told a vendor, 'We need 10 bar compressor outlet.' They heard '10 bara.' The difference is about 1 bar, roughly 14.5 psi, and it didn't show up until commissioning, when the downstream pressure was just short of the process requirement. The technician went quiet, and then we got a lesson in gauge versus absolute pressure.

If the gas being compressed is hydrocarbon-based, I also ask whether the package is designed and tested to API 619. It is not always the right spec for a simple plant air unit, but it gives both sides a clear language for design and testing.

What a K&N air filter taught me about compressor packages

I run a K&N air filter in my truck. I like it. But I would not put one on a compressor intake, no matter how good the adapter looks.

K&N filters are engineered for engines, where airflow and low restriction matter. A compressor package needs an intake filter with a known dirt-holding capacity and pressure drop over the maintenance interval. An automotive-style high-flow filter on a Howden roots blower or screw compressor can let fine particles reach the rotors or oil system. That can turn a $60 filter issue into a $15,000 overhaul. Not because K&N is bad; because it's the wrong tool.

The pool heater lesson: bigger is not a safety margin

Last year, I replaced a pool heater at home. I almost bought the largest model because I did not want to run out of heat. The contractor asked three questions: surface area, desired temperature rise, and local wind exposure. The mid-size heater was a better fit. The bigger one would have short-cycled and wasted gas.

A gas compressor is the same. I once had a vendor quote a larger Howden screw compressor than we needed, with the phrase 'room to grow.' That growth never happened. The compressor ran unloaded for months, and unloaded running still consumes power. The surprise wasn't the initial price difference; it was the operating cost at the end of the year.

Controls are the Honeywell thermostat part of the system

The control setup is where the Honeywell thermostat analogy kicks in. If you've ever looked up how to set Honeywell thermostat programming, you know the idea: set a schedule, not a single temperature. You don't leave it in 'hold' all year.

Compressor controls require the same approach. Set a pressure band (for example, 6.5 to 7.5 bar), a duty schedule, and a remote alarm. One facility we inherited had been running a large screw compressor 24/7 because nobody configured the timer. The room was empty at night, but the compressor wasn't. That was pure waste, and it wasn't the machine's fault—it was the setpoint logic.

If you're choosing between a Howden roots blower and a Howden screw compressor, ask about the control package before you ask about the paint color. Seriously, the mechanical reliability matters, but the controls are what make it cost money or save money.

Where I would push back

Not every gas handling job needs a Howden machine. Or, rather, not every job needs the machine I would guess first. If your process needs 40 bar or higher, a screw compressor may not be the right answer; a reciprocating compressor is often a better fit. If you need very high flow at low pressure and have floor space, a centrifugal blower might beat a roots blower. And if the duty is truly intermittent, a smaller packaged compressor could save you more than either option.

I am comfortable recommending Howden for the applications I have seen: low-pressure gas movement with a roots blower, continuous compression with a screw machine. But I would also tell you to get an independent engineering review if you are near the edge of the envelope. That is not a caveat; it is a way to keep the capital request from becoming a case study in what not to do. I still can't design a compressor package, and I probably never will. But I've learned to ask the questions that keep the engineers from having a bad day.