Pneumatic conveying of hydrated lime depends on the powder’s moisture history and flow behaviour as much as on the air source. Hydrated lime and quicklime can both move through enclosed pipelines, but they are different materials: hydrated lime is a fine calcium hydroxide powder, while quicklime is calcium oxide and may arrive as powder, granules or lumps. Their response to moisture, temperature and handling differs. Treating them as interchangeable can lead to unstable feeding, deposits, inaccurate dosing or excessive wear.
A roots blower supplies conveying air; it does not determine whether the lime will flow reliably. The selection depends on the required airflow and differential pressure, the feeder, the pipeline and the receiving equipment, as well as the machine’s permitted speed, power and discharge temperature. A sound design starts with representative lime data and a complete system pressure budget. Generic velocity figures or a catalogue-wide pressure limit cannot replace those checks.
Define the Lime and the Conveying Duty
A lime plant may handle limestone, quicklime, hydrated lime and process dust. Coarse limestone and large quicklime pieces are usually better suited to mechanical handling. Fine quicklime and dry hydrated lime are candidates for pneumatic transfer when their particle size, flow properties and route fit the system. A blower selected for quicklime conveying must also account for the product’s temperature and particle condition; the word “lime” alone is not enough to define the duty.
Quicklime is calcium oxide. When it contacts water, it forms calcium hydroxide and releases heat. Hydrated lime is made by controlling that reaction to produce a dry powder, but the stored product can still be affected by humid air and carbon dioxide. Moisture may promote caking, while carbonation can change the powder’s chemistry over time. The extent depends on the lime grade, exposure and storage conditions, so a short exposure should not be treated as equivalent to prolonged poor storage.
Before selecting equipment, identify the product grade, particle-size distribution, largest particles, moisture content, bulk density, temperature at the feeder, and flow behaviour after storage. Include the expected range in solids rate and whether the transfer is continuous or batch. A bulk-density figure by itself does not predict whether a powder will aerate, bridge, compact or discharge consistently. If the line is critical to production, test a representative sample or compare it with operating data from a genuinely similar material and route.
The route is part of the material duty. Record pipe bore and length, vertical sections, bends, branches, receiver and filter resistance, injection or discharge pressure, and the required peak rate. Include start-up, shutdown and restart conditions. A batch system may need a higher instantaneous flow than its average production rate suggests because the line must complete transfer within a limited part of the operating cycle.
Dry, fine lime is often considered for dilute-phase conveying, where a relatively low solids loading is carried in a moving air stream. That does not mean every lime product will behave the same way or that a roots blower is automatically suitable. Dense-phase operation depends on the material’s air retention and permeability as well as available pressure. If the process relies on low-velocity plugs or a high solids loading, confirm performance with material trials or dependable plant data before fixing the equipment concept.
For pneumatic conveying of hydrated lime, velocity must be based on the actual powder and the full line. The minimum stable velocity depends on particle size and shape, solids loading, pipe orientation, pressure and material condition. A single number cannot be applied to both positive-pressure and vacuum systems. Excessive velocity can increase pipe wear, product degradation and air demand; insufficient velocity can let solids settle and start a blockage. Use test data or a relevant operating reference, then retain a practical margin for changes in feed and air conditions.
Air volume also changes along a pipeline. As pressure falls, the same air mass occupies more volume and its local velocity rises. Check the gas state at the relevant pipe sections using absolute pressure and temperature in kelvin. When converting a reference or free-air flow to local volume, account for both pressure and temperature; do not use gauge pressure as though it were absolute. This prevents an apparently adequate nameplate flow from being mistaken for adequate conveying velocity everywhere in the line.
Control Air, Feeding and Injection
The pressure budget should include the feeder, straight pipe, bends, vertical lift, solids acceleration, receiver, filter and any downstream injection point. Assess normal operation and the highest credible steady duty. An air-only pipeline calculation can establish a reference, but material friction and feeder losses still have to be included. The blower’s operating point must be checked against the selected model’s curve and limits at the actual inlet conditions, not simply against a nominal pressure label.
A rotary valve is a common way to meter powder into a positive-pressure line and limit air leakage back into the hopper. Its leakage depends on pressure difference, clearances, speed, construction and wear. That leakage consumes part of the air available for conveying, so the supplier should include it in the duty calculation. As abrasive fines wear the rotor tips or casing, leakage can increase even while the blower continues to run normally. A change in feeder condition may therefore look like a conveying-air problem.
Hopper flow needs equal attention. Hydrated lime can bridge or consolidate during storage, while aeration can make it discharge too freely. The hopper outlet, wall geometry and any flow aid should be chosen for the tested powder and required rate. A rotary feeder, screw feeder or pressure-vessel arrangement may fit different duties; no feeder style is universally best. Where mass-rate accuracy matters, use a suitable gravimetric dosing arrangement and account for the effects of aeration on its measurement and control.
The conveying air must be dry enough for the product and the coldest part of the route. Quicklime reacts with water and releases heat; hydrated lime can take up moisture and carbon dioxide, encouraging caking or chemical change. A roots blower warms the air as it delivers it against pressure, but warming does not remove water. Unless water is removed through a dryer or condenses out of the gas, its moisture content remains essentially unchanged. The hot discharge may have low relative humidity, yet condensation can still form later when the gas cools.
For example, air containing about 10 grams of water per kilogram of dry air has a dew point near 14°C at atmospheric pressure. At 2 bar absolute, the same moisture content has a pressure dew point of roughly 25°C. This is an illustrative psychrometric calculation, not a specification for a lime line. It shows why a pipe that cools to 20°C could collect condensate at that pressure even though the air left the blower hot. Specify the required pressure dew point at the operating pressure and compare it with the lowest expected pipe temperature.
If the lime’s moisture sensitivity or site climate requires it, select an air dryer to meet the stated pressure dew point. Insulate or heat exposed pipe where the temperature profile requires it, prevent humid-air ingress during shutdown, and provide suitable condensate management upstream of the conveying line. A brief empty-line air purge before or after transfer can help only when it is consistent with the process and dust-control design. If product quality requires oil-free conveying air, state that requirement and verify the complete package arrangement; do not infer air quality from the blower mechanism alone.
Quicklime and hydrated lime can also differ in temperature and wear duty. Fine quicklime may arrive warm from upstream processing, while hard inclusions or coarser particles can increase impact at bends. Check the temperature rating of pipework, flexible connections, feeder seals and filters. Place inspection points at bends and other high-wear areas, and consider long-radius or replaceable wear sections where the material data justify them. Lowering velocity can reduce erosion, but it must not take the line below its stable conveying range.
Hydrated lime injection is a metered conveying duty in flue-gas treatment. Reliable pneumatic conveying of hydrated lime for injection depends on matching the feeder’s solids rate to a stable air stream. The blower supplies air to carry the powder through the line and injection lance. A roots blower can be considered when the required flow and pressure fit the selected model and the system’s thermal limits. The pressure available at the lance must exceed duct pressure plus the losses through the feeder and conveying route. The blower does not meter lime, and a steady air supply cannot compensate for an unstable feeder or a poorly distributed injection point.
For injection service, establish baseline line pressure at a known lime rate and monitor gradual changes. A rising pressure may indicate deposits, a partially restricted lance, a loaded filter or a change in feed behaviour. It is a signal to inspect the complete line, not a reason to raise blower speed without checking the operating limits. Purge and interlock logic should be designed around the process so that the line is cleared safely when lime feed stops.
Commissioning and Operating Limits
Commission the line with the intended lime grade and record a stable reference condition. Useful values include conveying-air flow, blower inlet and discharge pressure, discharge temperature, motor current, feeder speed or load, filter differential pressure and solids rate. Keep the reference tied to known operating conditions; pressure readings are difficult to compare if material rate, valve position, filter condition or ambient temperature has changed.
A typical sequence establishes conveying air before powder feed, then clears the line after feed stops. The actual sequence and interlocks depend on the feeder, receiver and process, so they belong in the approved operating procedure. After a long shutdown, check that powder has not compacted or caked around the hopper outlet. A surge of solids during restart can exceed the line’s stable capacity even though the blower itself is operating correctly.
Trend pressure at a constant or recorded feed rate. If pressure drop rises over time, inspect for moisture-related build-up, filter loading, restriction or a change in material condition. If the line becomes less stable while pressure remains similar, check feeder delivery, leakage, conveying flow and instrument calibration. A single gauge cannot identify every cause; comparing airflow, pressure, temperature and feed data gives the maintenance team a more useful diagnosis.
Lime dust is strongly alkaline and can injure eyes or irritate skin and airways. Keep transfer points enclosed, capture dust at loading and discharge, and follow the product safety data and site protective-equipment requirements. Quicklime needs additional care around water because hydration releases heat. Vacuum recovery may help contain spills because leakage draws air inward, but a vacuum cleaning system needs its own exhauster rating and dust-protection filters. A positive-pressure conveying selection does not automatically qualify a blower for vacuum duty.
There is no universal pressure ceiling that applies to every roots blower model. Confirm the selected unit’s permitted differential pressure, speed, absorbed power, temperature rise and discharge temperature for the required operating range. Also check site altitude, inlet temperature, control range and any limits imposed by the package. If the material duty requires more pressure or a thermal condition outside the verified envelope, revise the route, rate, pipe size or conveying concept before specifying the machine.
Glossary
Quicklime: Calcium oxide, usually produced by heating limestone; it reacts with water and releases heat.
Hydrated lime: Calcium hydroxide powder made by adding a controlled amount of water to quicklime.
Pressure dew point: The temperature at which water vapour in compressed or conveying air begins to condense at the stated pressure.
Dilute-phase conveying: A conveying mode in which a relatively low solids loading travels in a moving gas stream.
Differential pressure: The pressure difference across the blower or between the conveying line and its surroundings.
Loss-in-weight feeder: A gravimetric feeder that estimates material delivery from the decrease in hopper weight over time.