Three-Lobe Roots Blower for Wastewater Treatment | 15-45KW Classic Longtech Roots Blower

The SL series is a three-lobe roots blower — a roots-type positive-displacement rotary blower engineered for continuous industrial service, also known as a Longtech roots blower. If you run a wastewater treatment plant, manage a fish farm, or operate any process that depends on a steady supply of pressurized air, this roots blower is built for exactly that: reliable airflow, month after month, without oil contamination and without drama.

Here’s how it works: two interlocking three-lobe rotors spin inside a cast iron casing, trapping a fixed pocket of air with each revolution and carrying it from inlet to outlet—no internal compression. No valves. No oil injection into the air stream. The flow rate remains constant regardless of discharge pressure — a defining characteristic of a Roots rotary lobe blower and the reason it dominates aeration applications.

What Makes Three Lobe Blowers Better Than Two

Older Roots blowers used two lobes per rotor. They worked. But the airflow came in pulses — each lobe pair pushing a slug of air, then a gap, then another slug. That pulsing translates into vibration, and vibration turns into noise and pipe stress.

Adding a third lobe changes the geometry. The air moves in smaller, more frequent increments. Here’s the quantified difference:

Our rotors are CNC-machined as matched pairs. The clearance between the lobes and the casing wall is held tight enough to keep internal leakage low, but never so tight that thermal expansion becomes a problem. It’s a balance that comes from building these for over a decade.

SL Series Three-Lobe Roots Blower Specifications

We make twelve frame sizes, from the compact SL-040 (popular for small pond aeration and biofloc systems) up to the SL-400 for heavy industrial duty. The table below covers the full range. If you need something outside these parameters — different drive arrangement, special motor enclosure, custom paint for coastal environments — ask. We build to order.

How to read this table: Airflow ranges from minimum to maximum across the full speed band. A given unit at a fixed RPM delivers a specific flow rate — we’ll provide the exact performance curve when you inquire. The larger frames (SL-350 and SL-400) are typically spec’d for cement plants, power stations, and large municipal treatment works. If that’s your application, our applications engineer will walk you through the sizing directly.

Classic Three-Lobe Roots Blower Structure Diagram

Noise Levels

How to Size Your Roots Blower (STP Conversion)

Our performance tables are based on STP standard conditions (20°C, 10332 mmAq atmospheric pressure, 65% relative humidity). If your site conditions differ — and they usually do — you need to convert before selecting a model.

Flow rate conversion: Q₂ = Q₁ × (P₁/P₂) × (T₂/T₁)

Pressure conversion: Ps = [(10332 + P₂) / (10332 + P₁) − 1] × 10332

Worked example: A customer at 40°C ambient needs 5.16 m³/min with suction at −500 mmAq and discharge at +4260 mmAq. Converting to STP → 4.60 m³/min at 5000 mmAq → select SL-080, 1300 rpm, 10 HP (7.5 kW) motor.

A note on selecting the right model: always factor in piping losses. A blower that looks perfectly sized on paper can under-deliver if the discharge line is long or has multiple bends. We typically recommend a 10–20% headroom margin on both flow and pressure. If you send us your system layout, our engineer will check the numbers before we quote.

Three-Lobe Roots Blowers Applications

Wastewater Treatment

This is the biggest application by volume. In activated sludge processes — whether it’s a municipal plant or an industrial effluent treatment system — the microorganisms that break down organic waste need oxygen. Lots of it, continuously.

Our SL three-lobe rotary blowers feed air into the aeration basin through diffusers at the tank bottom. The steady discharge characteristic of a roots-type blower matters here: as the water level changes or the diffusers age and increase back-pressure, the airflow holds. That’s not something centrifugal blowers do well at these pressure ranges.

What our customers tell us matters most in this application: the blower runs 24 hours a day, 7 days a week, sometimes for years without a shutdown. Oil-lubricated bearings with a simple splash lubrication system keep things turning. No complicated oil pumps. No external cooling circuits. Just check the oil level once a week and clean the inlet filter.

Aquaculture (Fish & Shrimp Farming)

The same basic need — oxygen in water — drives the aquaculture market. Shrimp farms in Southeast Asia. Tilapia operations in Africa. Recirculating aquaculture systems (RAS) in Europe and North America.

For pond aeration, we typically see SL-040 through SL-100 selected. The small roots blower (1 hp to 5 hp range) sits on the pond bank, feeding air through a manifold to multiple diffuser lines. One unit can serve several ponds if the piping is laid out right. For biofloc systems — increasingly popular in shrimp farming across Thailand, Indonesia, and India — the SL-040 and SL-050 are the most common choices because biofloc tanks need continuous aeration to keep bacterial flocs suspended.

For RAS installations — indoor, high-density tank systems — the requirements get stricter. These facilities run 24/7 and can’t afford downtime. The oil-free discharge from our roots blowers is essential here: you don’t want oil mist in water that’s being recirculated through biofilters and back to the fish.

Also Available: Vacuum Configuration (SLV Series)

The same three-lobe rotor platform works as a vacuum pump — just by reversing which port connects to your process. If your application involves pneumatic conveying, vacuum packaging, or tissue paper processing, see our SLV Series Roots Blower Vacuum Pump →.

What the Blower Is Made Of

Knowing what’s inside the machine matters when you’re the one maintaining it. Here’s the full breakdown:

Everything runs wet-sump lubricated. The gear end has an oil bath that the gears dip into on every revolution. A splash disc flings oil up into the bearings. Dead simple. No external oil pump to fail. No cooling water connection needed. Just drain and refill the oil on schedule, and the machine keeps running.

Accessories We Recommend

A bare roots blower isn’t much use on its own. We stock a complete accessory ecosystem — everything ships pre-sized to match your model:

• Inlet silencer (Model I-050 ~ I-400 indoor / O-050 ~ O-400 outdoor) — Mounted on the suction side. Takes the edge off intake roar. Weatherproof outdoor versions available.
• Discharge silencer (Model D-050 ~ D-400) — On the pressure side. Combined with the inlet silencer, this brings noise down from “you need ear protection” to “you can have a conversation nearby.”
• Check valve (Model C-050 ~ C-400) — Prevents backflow when the blower stops. Simple swing-check design, cast iron body. If this isn’t installed and your discharge line is pressurized, water or process fluid can flood back into the blower.
• Relief valve (Model R-038 ~ R-125) — Spring-loaded, adjustable set point. If the discharge line gets blocked, this opens before anything breaks. Mount it between the check valve and the first downstream block valve.
• Flexible joint (Model JF/F-050 ~ F-400) — Rubber expansion joint that isolates blower vibration from your piping. Saves a lot of flange leaks and cracked pipes over time.
• Base-mounted vibration absorbers (Model UJ-200J ~ UJ-800J) — Rubber-in-shear mounts that sit between the blower frame and your foundation.
• Tube connector / T-joint (Model V-050 ~ V-400 / T-050 ~ T-400) — For flange connections and flow splitting.

Roots blower package option: We can ship the blower pre-assembled on a common baseplate with motor, belt drive, inlet/discharge silencers, check valve, relief valve, and flexible joints already mounted and aligned. You set it down, bolt it to the foundation, connect the pipes, and wire the motor. If you need a turnkey solution, ask about our packaged skid systems.

Three-Lobe Roots Blower Installation Basics

If you’re handling the installation of this three-lobe roots blower yourself, here’s the standard layout for a pressure application:

A few things that save headaches later:

• Put the inlet silencer somewhere it can pull clean, cool air. Not next to a dust source or a hot discharge pipe.
• The discharge relief valve must be between the check valve and the first block valve downstream. If someone closes the block valve while the blower is running, the relief valve is the only thing preventing over-pressure.
• Leave enough clearance around the blower to pull the rotors if you ever need to. It’s rare, but if it happens in year 6 of operation, you’ll appreciate the forethought.
• In coastal or high-humidity environments, specify corrosion-resistant paint and consider a weatherproof motor enclosure.

Inlet → Silencer → Flexible Joint → Blower → Flexible Joint → Check Valve → Relief Valve → Discharge Silencer → Process

Why Buy Roots Type Blower from Slovdson

We’re a roots blower manufacturer based in Ganzhou, Jiangxi, China — not a trading company. When you contact us, you’re talking to the people who design and build the machines.

• ISO 9001 certified — documented processes, consistent quality, traceable materials
• 12 models covering 0.15 to 362 m³/min — small roots blowers for aquaculture up to large frames for municipal treatment
• Spare parts ship within 48 hours for common items (bearings, oil seals, gaskets, filter elements)
• OEM and private labeling available — your brand on our hardware, your manual, your packaging
• English-speaking sales engineers who can size a roots blower from your process data — not just quote a price, but check the engineering first

We’ve sold these roots blowers into wastewater plants, shrimp farms, cement factories, and chemical processing facilities across Asia, Africa, the Middle East, and South America. Here’s one real example:

Case in point: A 50,000 m³/day municipal wastewater treatment plant in Southeast Asia runs six SL-200 units for their aeration basins. The blowers were commissioned in 2019 and have been on continuous duty since — routine oil changes, one bearing replacement at the 4-year mark, zero unplanned downtime. The plant operator’s feedback: “They just run.”

The machine that leaves our factory in Ganzhou is the same machine that runs for years in a treatment plant in Nairobi or a shrimp pond in Thailand. That’s the point.

Frequently Asked Questions

Q: What is a roots-type blower?

A roots-type blower is a positive-displacement rotary blower that uses two or three interlocking lobes (rotors) to move a fixed volume of air or gas from inlet to outlet with each revolution. Unlike centrifugal fans, the flow rate stays nearly constant as discharge pressure changes. They’re sometimes called Roots rotary lobe blowers, and they’re the standard choice for wastewater aeration, pneumatic conveying, and any process requiring steady airflow against varying back-pressure.

Q: How much does a roots blower cost?

Roots blower price depends on model size, operating pressure, motor specification, and accessories. A small SL-040 unit for pond aeration with a 1 hp motor starts at an accessible price point; a fully accessorized SL-300 package for a municipal plant is a capital equipment investment. We quote within one working day — tell us your flow, pressure, and power supply, and we’ll send a detailed price breakdown. For reference: small roots blowers (1–5 hp range for aquaculture) are typically our highest-volume price segment.

Q: What’s the difference between a two-lobe and three-lobe roots blower?

A three-lobe roots blower produces 6 air pulses per revolution vs. 4 for a two-lobe (twin-lobe roots blower). The more frequent, smaller pulses result in smoother discharge flow, lower pressure pulsation, less vibration, and 3–5 dB(A) less noise. Three-lobe is the current industry standard for most applications. Two-lobe designs are still available but mainly for low-RPM, non-critical duty or legacy replacement.

Q: Can a roots blower run 24/7?

Yes. The SL series is designed for continuous duty. The wet-sump splash lubrication system has no external pump to fail, and the bearings are sized for L10 life exceeding 40,000 hours at rated conditions. Many of our units have been running 24/7 in wastewater plants for 5+ years with nothing but scheduled oil changes and filter cleaning.

Q: Does a roots blower produce oil-free air?

Yes. The gas path through the rotors and casing is completely isolated from the gearcase lubrication system by labyrinth seals. No oil is injected into the air stream. This is essential for wastewater aeration (oil kills bacteria), aquaculture (oil kills fish), and food processing. The oil-lubricated parts are sealed on the gear side and never contact the process air.