Cement conveying with a roots blower depends on the cement’s flow behaviour, the conveying route, the feeder and the pressure limits of the selected machine. A process sketch may show only a hopper, feeder, pipe, air source and receiver, but each part consumes pressure or air capacity. The practical design task is to prove that the complete system can deliver the required cement rate while maintaining a stable transport condition and remaining within the blower’s continuous operating envelope.
Cement Behaviour and Conveying Regimes
Cement is a fine powder that can retain air and change its flow behaviour with grade, moisture, storage and handling. Aerated cement may discharge readily from a hopper, while compacted product can resist flow or form an irregular feed. These differences affect the solids entering the line before they affect the blower. A useful design basis therefore identifies the actual cement grade, representative particle data, bulk density, moisture, temperature, storage history and required mass rate. The name “cement” alone does not define a predictable conveying duty.
Once cement enters the pipe, gas flow must carry it without creating an unstable deposit. At higher conveying velocities, particles travel mainly in suspension. At lower velocities, some cement systems can move in a sliding or moving bed. The change between these regimes depends on material, solids loading, pipe diameter, pressure, route and feeding method. Dense phase is not a fixed property of a blower or pipe; it describes behaviour demonstrated for a particular material and set of conveying conditions.
The numerical values sometimes used to discuss cement conveying need the same caution. In one 53 mm bore test line, 50 m long with nine 90-degree bends, the reported minimum inlet velocity for suspension was about 11 m/s; sliding-bed movement was reported at velocities down to about 4 m/s. Solids loading ratios near 40 in suspension and above 100 in sliding-bed conditions, along with a pressure gradient around 20 mbar per metre, are observations for that cement and test arrangement. They are not universal design constants for another cement grade or plant route.
These values can help explain why line length and available pressure matter, but they should not be extrapolated as a simple one-to-one rule. As the route becomes longer or more complex, friction, material acceleration, vertical lift, bends and equipment losses consume more of the available differential pressure. A lower pressure budget per metre may require a lower solids rate or a different transport regime, yet the relationship depends on the material and the line. Equivalent length is useful for screening; the final calculation must represent the actual fittings, elevations and operating cases.
Designers should establish the transport condition using representative material data, a validated calculation, a conveying trial or comparable operating evidence. A short test at high pressure does not automatically predict a long line at lower pressure. Likewise, a stable dilute-phase result does not prove that the same system will operate as a low-velocity dense-phase line. If the project depends on a narrow velocity margin or very high solids loading, confirm that behaviour before equipment selection.
Pressure Limits, Feeder Leakage and Line Resistance
Pressure capability is model-specific. A nominal figure such as 1 bar gauge may be useful as a rough screening reference, but it is not a universal roots blower limit or an equipment selection value. The selected machine’s continuous data govern the allowable differential pressure at the required speed and inlet condition. The review must also check absorbed power, discharge temperature, speed range and any package limits. The maximum permitted value is the first applicable limit reached, not simply the pressure number in a general product description.
The specification should also define the required conveying-air quality. The possibility of lubricant carryover depends on machine construction, seals and package arrangement, so it should be confirmed for the selected configuration instead of inferred from the blower principle. Inlet filtration protects the machine from ambient particles; it does not by itself establish the gas quality at the process connection. Where cement purity or downstream equipment imposes a limit, state that requirement and request documented confirmation.
Build the pressure budget from the blower inlet to the receiver and include every component that adds resistance. The inlet filter and silencer reduce inlet pressure; the discharge pipe, valves, bends and feeder add losses; the conveying line adds gas and solids pressure drop; and the receiver filter must pass the conveying air at its expected clean and loaded conditions. Vertical lift, material acceleration and outlet pressure also belong in the system calculation. The normal operating case and the maximum credible case should be compared with the machine’s verified operating limits. A blocked line is an upset condition, not a design point to be overcome by selecting more pressure.
The feeder has a direct effect on the air budget. A rotary valve can meter cement into a positive-pressure line and provide a pressure boundary, but it is not airtight. Leakage can pass through returning empty pockets, rotor-to-casing clearances and shaft seals. The amount depends on pressure difference, rotor dimensions, clearance, speed, wear and material condition. It must be estimated from the actual feeder data or measured where appropriate, then added to the net air requirement. A blower sized only for the pipeline flow can leave the line below its required conveying velocity.
An illustrative valve calculation shows why a generic allowance can be misleading. For a 300 mm diameter and 300 mm long valve operating at 20 rev/min with 0.2 mm clearances against 0.6 bar gauge, one estimate gives about 1.2 m³/min through clearances and 0.4 m³/min by pocket carry-over at pressure. Converted to free-air terms, the combined loss is about 2.6 m³/min. If the line itself needs 8 m³/min of free air, that example would put leakage near one third of the net line flow. These figures describe that assumed valve and pressure condition; another valve or worn clearance can produce a different result. They show why measured or calculated leakage is preferable to automatically adding 10–15 percent.
As clearances wear, leakage can rise while the blower remains at the same speed. The air left for conveying then falls, so a line can approach blockage without any change to its nominal blower setting. A vented feeder arrangement can direct leakage away from the hopper, but the vent path and filter must be included in the design. Feeder choice also sets an application boundary: venturi devices are generally associated with low-pressure, light-loading duties; low-pressure blow tanks may suit selected abrasive materials; screw-based pressure systems occupy a different pressure range. The correct arrangement follows the required duty, not a universal ranking of feeder types.
Preliminary Sizing Example
Consider a preliminary example using a 100 mm internal-diameter pipe. Assume that the conveying line needs 140 kPa absolute at its inlet, equivalent to about 0.4 bar gauge relative to 101.3 kPa atmospheric pressure. Assume a line-entry air temperature of 40°C, or 313 K. For illustration, take an inlet velocity 20 percent above the test-specific 11 m/s suspension value, giving 13.2 m/s. That velocity is an example input, not a recommended minimum for every cement.
The free-air volume referred to 15°C and 101.3 kPa can be estimated from the line-entry pressure, bore, velocity and absolute temperature:
V₀ = 2.23 × p₁ × d² × C₁ / T₁
Here, p₁ is in kPa absolute, d is in metres, C₁ is in metres per second and T₁ is in kelvin. The constant gives volume in cubic metres per second on the stated reference basis. Substituting the example values gives 2.23 × 140 × 0.1² × 13.2 / 313 = 0.132 m³/s, or approximately 7.9 m³/min of free air for the line.
Using a reference-air density of about 1.225 kg/m³, that volume corresponds to an air mass flow near 0.16 kg/s, or 580 kg/h. If a solids loading ratio of 15 kg cement per kg air is assumed for this example, the implied cement rate is approximately 8.7 tonnes per hour. The loading ratio is an assumption, not a demonstrated capacity. It must be supported by the actual material and conveying method before the rate is used as a guarantee or purchasing basis.
If gas temperature is treated as constant across the line, the outlet velocity estimate at 101.3 kPa absolute is 13.2 × 140 / 101.3, or about 18 m/s. This is a simplified estimate. The final outlet velocity should use local absolute pressure and gas temperature at the outlet. The calculation also stops at the line-entry condition: it has not yet added the pressure loss through the discharge pipe, feeder, bends, vertical sections, receiver or filter. Therefore, 140 kPa is not automatically the blower discharge pressure.
Rotary-valve leakage must be added to the net line flow before selecting the blower. If a preliminary spreadsheet applies a 15 percent allowance to 7.9 m³/min, the result is about 9.1 m³/min of free air. That allowance is only a placeholder. The valve example above indicates that leakage can be substantially higher under some dimensions and pressure conditions, so the actual feeder data should replace the percentage before a final selection is made.
A separate power estimate illustrates the calculation but does not complete the selection. For an intake flow around 0.15 m³/s and a pressure ratio of 1.4, ideal isentropic compression power is approximately 5 kW. If an illustrative isentropic-to-shaft efficiency of 86 percent is assumed, shaft power is about 6 kW before project-specific drive and package considerations. This pressure ratio is not derived from the line-entry value above; a real blower discharge pressure must cover line-entry pressure plus all upstream losses and leakage effects. Discharge temperature must come from the selected model’s data at the actual operating point rather than from a generic temperature estimate.
The arithmetic shows how velocity, pressure, bore and loading assumptions relate. It does not prove that the example line can convey 8.7 tonnes per hour. That requires the total pressure-drop calculation and validation of the cement behaviour over the actual distance and layout. If the calculated duty exceeds the selected model’s verified limits, the design needs a lower rate, a shorter or larger-bore route, an intermediate transfer stage or a different conveying system. A general pressure figure should never be used to force the calculation to fit a preferred machine.
Moisture, Blockage and the Selection Review
Water can cause cement to adhere or build up on pipe walls. Compression heats the air and can reduce its relative humidity, but it does not remove the water vapour. If the gas cools downstream below its pressure dew point, condensate can form in the pipe or receiver. For illustration, air carrying around 10 g of water per kg of dry air has a dew point near 14°C at atmospheric pressure and roughly 25°C at 2 bar absolute. The relevant value depends on the actual moisture content and pressure, so the design should specify air quality and assess the coldest parts of the route.
Exposed piping may need insulation or another approved method of controlling condensation. Low points should be reviewed for liquid accumulation, and drains should be used only where the system is designed for them and maintenance can remove condensate without introducing water or cement into the line. If moisture has entered the system, a purge may be appropriate only when the process procedure, dust collection and receiving equipment support it. Prevention is more reliable than relying on operators to clear a wet cement deposit after it forms.
Early warning comes from comparing readings at consistent operating conditions. Record an air-only pressure baseline at a stated flow, inlet condition and valve arrangement. During conveying, track blower inlet and discharge pressure, line pressure at useful points, flow, feeder rate, filter differential pressure and motor load. A rising pressure at the same air flow can indicate increasing resistance from deposits, filter loading, a partly closed valve or another restriction; it does not by itself identify the cause. A falling conveying flow with stable blower speed may point to wear, inlet restriction or feeder leakage. The trend should guide inspection rather than trigger an automatic speed increase.
Several common errors reduce the margin to blockage. Use absolute pressure in gas-density and velocity calculations; gauge pressure alone understates the actual pressure. Convert temperature to kelvin when using gas relationships. Do not exceed the blower’s stated differential-pressure, speed, power or temperature limits, even briefly. Confirm a cold-start case because lower inlet temperature changes gas density and can reduce velocity at a given volumetric flow. Finally, check the feed rate after a shutdown: settled cement may discharge differently from aerated material, and an overfed restart can exceed the line’s stable capacity.
The required operating margin should reflect measurement uncertainty, feed variation, filter loading and expected wear. A design barely above a tested minimum velocity can block after a modest change in feed rate or air delivery. A larger margin may improve robustness, but excessive velocity increases pressure loss, power demand and abrasion. The margin must be selected with the material and route data rather than copied as a universal percentage. Commissioning should record stable baseline values for each approved operating mode and define what action follows a pressure or flow deviation.
The technical enquiry should state cement type and grade, representative material data, required mass rate, continuous or batch operation, cycle time, pipe route and bore, bends, vertical lift, feeder type, estimated leakage, receiver and filter data, blower-inlet flow reference, inlet temperature and pressure, site altitude, normal and maximum credible differential pressure, control range, air-quality requirement and required accessories. For batch transfer, distinguish the active conveying period from the full cycle so the line rate is not understated by averaging over fill, switching or purge time. The enquiry should identify who verifies conveying capacity, filter performance and the system pressure calculation, and who is responsible for each guarantee. If a value is unavailable, list it as an open item instead of estimating it from a generic cement description.
The blower’s role is to supply the specified air at the required operating point. The conveying system designer remains responsible for proving that the chosen cement, feeder, line and receiver can meet the plant rate. Cement conveying with a roots blower is a sound concept only when material performance, feeder leakage, filter resistance and model-specific limits all close within the same pressure budget. If any one of those inputs is unverified, the next step is a material test or a revised system calculation, not a larger assumed blower rating.
Glossary
Differential pressure: Difference between blower discharge and inlet pressure at the stated operating condition.
Free air: Air volume converted to an agreed reference pressure and temperature.
Solids loading ratio: Mass of conveyed cement divided by mass of conveying air.
Rotary-valve leakage: Air passing through pockets, clearances and seals across a rotary feeder.
Equivalent length: A method of representing fittings and route features as added straight-pipe resistance.
Pressure dew point: Temperature at which water vapour begins to condense at the stated gas pressure.