Grain Bin Capacity, Aeration Fan and Drying Time

Capacity is derived from a cylinder and cone: $V=A(H+h_p/3)$, $A=\pi D^2/4$. Geometric US dry bushels are $V/0.03523907016688$; the inventory correction multiplies that count and the reference bulk mass.

Corn pressure drop follows CIGR Handbook Vol. IV, printed p. 26, Eq. (1.2): $\Delta p_g=R_p H_r a u^2/\ln(1+bu)$, with $u=Q/A$, $a=20700$ Pa·s²/m³, $b=30.4$ s/m and $H_r=H+h_p$. Purdue AE-106 supports a normal-fill resistance factor of 1.5. This factor is distinct from inventory packing. Floor/duct losses use an independently entered reference flow and scale as $\Delta p_e=\Delta p_{e,*}(Q/Q_{ref})^2$. Positive published fan points are interpolated linearly; parallel fan flows add at equal pressure. A bracketed root solves fan pressure minus system pressure. The curve is never extrapolated to claim an operating point.

The Chung–Pfost dry-basis fit is $X=0.339-0.059\ln[-(T+30.205)\ln(RH)]$ (CIGR printed p. 25). The modified Henderson cross-check is $X=[-\ln(1-RH)/(8.6541\times10^{-5}(T+49.81))]^{1/1.8634}/100$ (open Agricultural Engineering paper, Eq. 4). Both use °C and fractional RH; convert to wet basis by $100X/(1+X)$ and use the higher prediction as the target feasibility screen. These are published empirical fits, not a claim of compliance with ASABE D245.

Dry matter is conserved: $m_d=m_i(1-x_i)$, $m_f=m_d/(1-x_f)$ and water removal is $m_i-m_f$. FAO-56 Eq. 11 gives saturation pressure. Ideal-gas humidity ratio and moist-air enthalpy give an adiabatic saturation ceiling. A second ceiling comes from wet-zone grain equilibrium: invert both moisture fits at the initial dry-basis grain moisture, use the lower equilibrium RH at each temperature, and solve for its intersection with the inlet enthalpy line between the saturated-exhaust and inlet temperatures. The resulting humidity gain divided by the saturation gain is the displayed upper pickup fraction. The requested fraction must be strictly below it so the exhaust EMC stays below wet-zone grain moisture; excessive pickup is rejected rather than silently clipped. For feasible inputs, the water rate is dry-air mass flow × saturation humidity gain × requested pickup fraction. Water divided by this rate gives front-passage hours. The two-zone visualization sets the lower region at target moisture and upper region at initial moisture, and inverts processed volume to obtain front height. These last front and time relations are derived here from the stated water balance.

Drying airflow guidance follows Purdue AE-106 Table 3: 1, 2 and 3 cfm/bu for corn in the 20–22%, 22–24% and 24–26% wet-basis bands. At the shared boundaries 22% and 24%, use the higher band conservatively. For initial moisture below 20%, 0.5 cfm/bu is only a preliminary lower drying-airflow screen from CGC/PAMI. No Purdue recommendation is extrapolated above 26%. The delivered flow is compared with this moisture-dependent guideline.

Full perforated floor, upward flow, one cylindrical fill and a full-width conical peak; no hopper or central drawdown. Uniform reference wet bulk density and dry matter, with the inventory packing correction applied once. The whole floor sees peak depth for the conservative resistance calculation. Grain resistance is fixed during drying.

Fan curves are the CGC/PAMI archived Grain Guard examples, updated 11 March 2021. Only positive tabulated points are used (2–6, 2–8, 2–10 or 2–12 in water, depending on model). The selected operating point must lie within that interval. Fans in parallel are identical and share a plenum. Shaft power uses an assumed static efficiency; actual electrical input and motor overload require manufacturer data.

Constant inlet air at standard atmospheric pressure (101.325 kPa), no separate heater model, no weather sequence. Air temperature and RH are specified at the grain inlet. Pickup fraction is an explicit scenario assumption (default 0.65), not a crop-specific experimental calibration. It is relative to the adiabatic saturation humidity gain and must remain below the wet-grain equilibrium bound. The undried upper zone stays at the entered initial moisture; both inlet and exhaust driving-force checks apply. The lower zone is idealized at the target moisture, without overdrying or a finite transition band. Mechanical/handling shrink and dry matter loss are excluded.

The duct loss and its reference flow describe a fixed physical system. Changing the desired airflow does not alter that system curve. Front height uses the original fill geometry and neglects bed settlement; water shrink is mass loss, not predicted volume contraction.

If the fan curve has no bracketed intersection, the boundary fallback is internal only. All operating flow, pressure, shaft power, velocity, air mass flow and drying-rate results are unavailable; required design duties remain calculable.

For preliminary capacity and shelled-corn aeration/drying comparisons. The corn resistance fit is used within a superficial velocity range of about 0.0056–0.304 m/s; outside this range, change fan capacity or bin dimensions and verify against measured pressure. Input air is limited to 10–35 °C and 20–90% RH. Equilibrium moisture depends on variety and sorption history; approach to equilibrium slows real drying substantially. A target within one moisture percentage point of either equilibrium estimate needs special caution.

Drying hours are a constant-air, grain-equilibrium-constrained water-carrying-capacity estimate, not a validated field drying forecast. A requested pickup that would make the exhaust rewet the wet zone produces no drying-time result; reduce it below the displayed upper fraction or change inlet conditions. Low cooling airflow can produce a numerical time but is not adequate proof of drying before spoilage. Peaked grain and fines cause nonuniform flow; level and inspect from outside. Verify a chosen fan with a full-bin manometer reading and its exact certified curve. This tool does not establish safe storage time, safe entry, vent structural capacity or bin structural strength.

Never enter flowing grain, walk down grain, or stand on a grain bridge. Necessary entry belongs to trained personnel following the site's isolation, atmosphere-testing, attendant and rescue procedures. A running fan does not establish a safe atmosphere. Installation and electrical work belong to qualified persons.

Purdue's deep-bed drying guidance prefers 12–16 ft depth and recommends 20 ft maximum. The tool warns above 20 ft, above 26% initial moisture, or when delivered airflow is below the moisture-dependent guideline. These warnings do not constitute a safe-storage-time assessment; wetter grain or deeper beds need a dedicated drying design.