Most RFQs say: “Mixer 0–60 rpm” or “1.5 kW motor.”
That’s not enough. The mixer that actually works depends on torque at low speed, not just motor power.
Under-spec torque → stalled mixing, settling, inconsistent extraction
Over-spec torque → unnecessary gearbox cost, long lead time, and higher maintenance
This guide shows what buyers should specify so suppliers can design correctly—without you paying for oversized hardware.
If you’re choosing impeller style first, read: impeller selection by solids and viscosity
1) Power vs Torque (Buyer-friendly explanation)
1.Power (kW)
Power is the rate of doing work. In mixing, it relates to:
how much energy you can put into the fluid per time
heat-up from mixing (usually small compared with jacket heat)
how strong the turbulence and circulation can be (in low viscosity)
2.Torque (N·m)
Torque is rotational “twisting force.” It controls:
whether the mixer can turn thick fluids at low RPM
whether it can restart under load
whether it can handle viscosity spikes and high solids
3.Key buyer truth
At the same power, torque changes with speed: Torque increases as RPM decreases (for a given power). That’s why “low-speed high-torque” mixers need gearboxes.
2) When Torque Matters More Than Power (Most extraction slurries)
Torque becomes the critical spec when you have:
high solids loading
viscosity that rises during extraction (gums/resins)
low-speed impellers (anchor, helical ribbon, multi-stage)
cold operation (viscosity high)
risk of clumping and dead zones
If you commonly run high solids, see: high-solids mixing fixes
3) What to Specify in an RFQ (So you don’t overpay)
Instead of guessing a motor size, specify the mixing duty and constraints. Suppliers can then select power/torque properly.
3.1 Your “must provide” inputs (buyers)
Working volume and liquid level range
Solids loading (% w/w or g/L) + particle size range
Viscosity behavior
starting viscosity
peak viscosity (or “viscosity rises sharply”)
temperature range (viscosity is temperature dependent)
Mixing objective
off-bottom suspension? complete suspension? wetting only? gentle mixing?
Operating speed range (if you have preference)
Shear sensitivity (avoid fines generation?)
If you don’t know working volume properly, start with: extraction tank sizing (working volume + headspace)
3.2 Ask suppliers to respond with these outputs
Require the supplier to state:
recommended impeller type(s), diameter, and stages
speed range (RPM)
required torque at the shaft (N·m) at operating RPM
motor power (kW) and gearbox ratio
seal type and design limits (especially with solvents)
This makes quotes comparable and prevents “cheap mixer that doesn’t work.”
4) Practical Buyer Rules to Avoid Overpaying
Rule 1: Design for peak viscosity, not initial viscosity
Many herbal slurries start watery and then thicken. If you size for the start, the mixer stalls later.
Rule 2: Use pumping (large diameter, lower RPM) before using shear (high RPM)
If your problem is suspension and circulation:
larger diameter axial impeller at lower RPM usually improves mixing with less fines
high RPM often increases fines and filtration problems
Rule 3: Don’t demand huge speed ranges unless you need them
Wide speed range adds complexity and cost. If your process needs “two stable regimes” (wetting vs extraction), specify those two RPM windows.
Rule 4: Confirm restart torque (worst-case scenario)
Ask: can the mixer restart if the slurry thickens or settles overnight? If not, you may need:
higher torque margin
different impeller style
process change (don’t allow settling)
Rule 5: Link mixer design to downstream filtration
Over-shear can make filtration worse by generating fines. If filtration is already tight, specify a “fines-sensitive” requirement.
5) How Power/Torque Affects Extraction Performance (What buyers feel)
Under-torqued systems cause:
bottom settling and dead zones
inconsistent yield and quality
longer extraction time (cycle time increases)
more operator intervention (manual stirring)
Over-torqued systems can cause:
unnecessary CAPEX
longer lead time (gearbox selection)
higher maintenance costs
sometimes excessive shear if misused
If your goal is faster throughput, read: 7 engineering levers that shorten extraction cycle time
6) Troubleshooting “Our Mixer Is Too Weak” vs “We Overpaid”
Symptom A: Mixer stalls or trips on overload
Most likely:
peak viscosity underestimated
solids loading too high for current impeller
RPM too low for current impeller diameter
Fix:
confirm viscosity curve and solids %
change impeller style or add stage
increase gearbox torque (not always motor power)
Symptom B: Mixer runs but solids settle
Most likely:
flow pattern wrong (vortex without circulation)
impeller too high or too small
Fix:
adjust impeller placement/diameter
add baffles
consider axial-flow stage
Symptom C: Mixer “works” but filtration got worse
Most likely:
over-shear generating fines
Fix:
reduce tip speed
increase impeller diameter and lower RPM
improve staged filtration
7) RFQ Add-ons That Improve Quote Accuracy
If you want the whole line proposal, include:
solvent system and safety scope
reflux requirement
filtration and recovery constraints
documentation level and FAT expectations
A copy/paste template: herbal extraction line quotation checklist
FAQ (with answers)
1. Is motor power (kW) enough to specify a mixer?
No. For extraction slurries, torque at low RPM is often the limiting factor. Always request torque and gearbox ratio.
2. Why does a low-RPM mixer cost more even if power is similar?
Because low RPM requires a gearbox that delivers high torque. Gearboxes and seals are cost drivers.
3. What’s the best way to avoid buying an oversized gearbox?
Provide the right inputs (peak viscosity, solids %, mixing goal) and ask for the calculated required torque at operating RPM—then choose a reasonable safety margin, not an arbitrary “biggest possible.”
4. Can a stronger mixer shorten extraction time?
Yes, if the bottleneck is mass transfer due to poor mixing. But if filtration or recovery is the bottleneck, a stronger mixer won’t increase plant throughput.
5. Why do mixers fail when we increase solids loading?
Because torque demand rises sharply with viscosity and solids. High solids can change the flow regime completely.
Want a torque/power recommendation for your extraction slurry?
Send solids %, particle size, viscosity behavior (start/peak), temperature range, and mixing objective. We’ll recommend RPM range, impeller style, and torque targets for RFQ.