Psychrometrics in agriculture: greenhouses, grain and crop drying

Every agricultural decision involving air, whether it is ventilating a greenhouse, the timing of the harvest, the dryer settings or the conditions in a grain silo, has a psychrometric basis. Air that is too humid causes mold; too dry harms plants or cracks the product. The difference from industry lies in the sorption behavior of biological materials, where grain, fruit or wood actively take up and release moisture according to their equilibrium with the surrounding air.

Plant transpiration and greenhouse humidity

Plants release water vapor through their leaves by transpiration. Its intensity depends on the vapor pressure deficit (VPD), that is on the difference between the saturation vapor pressure at the leaf temperature and the actual partial vapor pressure in the air:

$$\text{VPD} = p_{ws}(T_{\text{leaf}}) - p_w(\text{air})$$

A high VPD (dry air) causes intense transpiration, stomatal closure and plant stress. A low VPD (humid air) limits transpiration but encourages fungal diseases (grey mould, Botrytis). The optimum VPD for most vegetables and flowers is roughly 0.5–1.3 kPa, i.e. about 20–26 °C / 60–75% RH. Each VPD value corresponds to a specific combination of temperature and relative humidity on the h-x diagram.

Equilibrium moisture content (EMC) and when grain stops drying out

An agricultural product exchanges moisture with the air until it reaches its equilibrium moisture content (EMC) — the material moisture at which the partial vapor pressure in the material equals that in the air and the exchange stops. EMC depends on temperature and relative humidity and is described by sorption isotherms. The ASABE D245.6 standard contains isotherms for 40 commodities. Example for wheat: at 20 °C and 65% RH, the EMC is about 13% (wet basis), which corresponds to a safe storage moisture.

Safe storage moisture for grain

Grain stored wetter than the equilibrium moisture content that is safe against microbial growth spoils quickly:

• Short-term storage (up to 6 months): wheat < 14%, corn < 14%, sunflower < 9% (wb)
• Long-term storage (over winter): wheat < 13%, corn < 13% (wb)

These values correspond to an air RH of about 60–65% at 15–20 °C. Modern silos are therefore fitted with controlled ventilation and run it only when the air is drier than the grain, a decision that can only be made with psychrometric data.

Post-harvest grain drying on the h-x diagram

Harvested corn has a moisture of 25–30% (wb), and for safe storage it must drop to 13–14%. Drying air: typically 80–100 °C, inlet φ < 5%.

The amount of water to evaporate from 1 t of corn (from 27% to 14% wb):

$$m_{\text{water}} = 1000 \cdot \frac{0.27 - 0.14}{1 - 0.14} \approx 151\ \text{kg water/t}$$

The amount of air needed at $\Delta x = 18 - 2 = 16\ \text{g/kg}$:

$$m_{\text{air}} = \frac{151}{0.016} \approx 9\,440\ \text{kg air/t}$$

Heater capacity for an 8-hour drying run and $\Delta h \approx 80\ \text{kJ/kg}$: $\Phi \approx 26\ \text{kW}$ per tonne of corn per hour. (The exact EMC and $\Delta h$ values depend on the variety, so verify them in ASABE D245.6 or with the dryer manufacturer.) The principles are the same as for industrial drying.

Greenhouse systems and their ventilation vs. cooling

In a greenhouse, the sun generates enormous heat gains, so in summer, without cooling, the temperature can rise to 50–60 °C. Natural ventilation (side and roof vents) is the cheapest solution, but at outdoor temperatures above 30 °C it is not enough. Evaporative cooling (a porous pad and a fan) is widespread in greenhouses. In principle, water evaporates on a cellulose pad, and the air is cooled and humidified along an isenthalp. Effective in a dry climate, less so in a Central European summer with a higher inlet RH. For the details, see the article Evaporative cooling.

Frequently asked questions

What is VPD and why does it matter more to growers than relative humidity? VPD is the difference between the saturation and the actual vapor pressure — the “pull” of the air for water from the leaves. It governs transpiration directly, whereas relative humidity is ambiguous without the leaf temperature.

How do I know when it is safe to ventilate a grain store? When the outdoor (or blown-in) air is in equilibrium with a drier grain moisture — that is, its EMC is lower than the current moisture of the grain. This is determined from the air temperature and relative humidity using sorption isotherms.

Does grain drying differ from industrial wood drying? Physically, no — it involves the same psychrometric relationships. What differs is the temperatures, the drying rate and the product’s sensitivity to cracking or loss of germination.