Industrial Drying and Psychrometrics: Dryer Design on the Chart

Industrial air drying is a psychrometric process: hot dry air comes into contact with wet material, takes up the evaporated water and leaves the dryer as more humid air. Setting the temperature, humidity and flow rate of the drying air correctly means minimizing energy use, maximizing the drying rate and not damaging the material — and the psychrometric chart is built for exactly these calculations.

The two phases of drying and their psychrometric profile

The constant-rate drying phase

There is enough free water on the surface of the material. Drying proceeds as evaporation from a free water surface — the rate is constant and depends only on the air conditions (temperature, humidity, flow velocity). The air takes up moisture at an approximately constant wet-bulb temperature, that is approximately along a constant-enthalpy line. All the energy goes into evaporation and the material temperature stays low (close to the wet-bulb temperature).

The falling-rate drying phase

The free water has been removed from the surface, and the remaining moisture migrates from the interior of the material to the surface by diffusion. The drying rate falls and the material temperature rises (approaching the dry-bulb temperature). This phase is critical for quality — drying too fast causes checking in lumber or a hard crust on food. It is controlled by lowering the temperature or raising the humidity of the drying air.

The psychrometric parameters of the drying air

• Supply-air temperature $t_{\text{sup}}$ — the higher it is, the faster the drying, but it is limited by the sensitivity of the product. Lumber 120–195°F depending on the species and the kiln schedule, food 105–160°F, paper 210–300°F.
• Relative humidity of the exhaust air $\varphi_{\text{ex}}$ — determines the utilization of the drying air. A higher $\varphi_{\text{ex}}$ means less air is needed, but slower drying.
• Air flow rate $\dot{m}$ — a larger flow = faster drying, but higher operating costs.

Calculating the evaporated water and the energy balance

The amount of evaporated water:

$$\dot{m}_{\text{water}} = \dot{m} \cdot \Delta W, \qquad \Delta W = W_{\text{ex}} - W_{\text{sup}} \quad \text{[lb/lb dry air]}$$

(if $\Delta W$ is taken from the chart in gr/lb, divide by 7,000 before substituting)

The heater capacity, which only preheats the air at a constant humidity ratio:

$$Q_{\text{heat}} = \dot{m} \cdot (h_{\text{sup}} - h_{\text{out}})$$

This, however, is not the total energy demand of the dryer. The heater merely conditions the air — the latent heat of the water the air carries away comes on top of it in the balance. The total heat input follows from the exhaust state and from the flow of fresh air $\dot{m}_{\text{f}}$, that is the air which actually passes through the dryer and leaves, not the recirculated loop:

$$Q = \dot{m}_{\text{f}} \cdot (h_{\text{ex}} - h_{\text{out}})$$

The specific energy use per pound of evaporated water is $e = Q / \dot{m}_{\text{water}}$ [Btu/lb]. Conventional dryers without heat recovery typically land at 1,290–2,580 Btu/lb. The physical floor is the latent heat of water, roughly 1,030 Btu/lb — if you come out below that, something is missing from the balance.

Recirculation: what it actually saves and what it does not

A large part of the operating savings comes from recirculating the exhaust air, which is warmer than the outdoor air — venting it out and replacing it with cold fresh air would be wasteful. Recirculation returns part of the exhaust air to the inlet, where it mixes with fresh air and is reheated. On the chart this is a mixing point on the line joining the exhaust and the outdoor state; where the point sits on that line corresponds to the recirculation fraction $r$.

How much this saves is a common source of error. If only the fraction $(1-r)$ of the air is vented, the heat input falls exactly in proportion to $(1-r)$ — but so does the amount of water evaporated, because the supply air is already pre-humidified and can take up less further moisture. The charge takes proportionally longer. Substitute the mixing into the balance and the $(1-r)$ cancels out:

$$e = \frac{Q}{\dot{m}_{\text{water}}} = \frac{h_{\text{ex}} - h_{\text{out}}}{W_{\text{ex}} - W_{\text{out}}}$$

The specific energy per pound of evaporated water is therefore completely independent of the recirculation fraction — it is governed only by the exhaust and outdoor states. Recirculation reduces the heater capacity, and with it the size of the heat source, but on its own it saves no energy in drying the charge.

The same formula shows where the savings do lie: in a more humid exhaust. Every pound of air leaving the building should carry out as much water and as little heat as possible. And that is exactly what recirculation is for — it is what keeps the humidity in the kiln high without bringing the drying to a halt.

The limit is the buildup of moisture — the higher the humidity ratio of the supply air, the weaker the driving force of drying and the longer the charge takes.

Example: a lumber kiln from 60% to 12% moisture content

Lumber moisture content is given on a dry basis (referenced to the oven-dry mass, not the total wet mass). For a charge with a dry mass of 13,750 lb (i.e. 22,000 lb of green lumber at an initial moisture content of 60%), the initial water content is $0.60\cdot13{,}750=8{,}250$ lb, and the target at 12% is $0.12\cdot13{,}750=1{,}650$ lb — so 6,600 lb of water has to be evaporated.

The drying air circulates through the kiln at 79,000 lb/h of dry air: supply 140°F / φ = 15% ($W \approx 133\ \text{gr/lb}$, $h \approx 55.0\ \text{Btu/lb}$), exhaust 140°F / φ = 45% ($W \approx 425\ \text{gr/lb}$, $h \approx 101\ \text{Btu/lb}$), so $\Delta W \approx 292\ \text{gr/lb} = 0.0417\ \text{lb/lb}$. The outdoor air is 68°F / φ = 60% ($W \approx 61\ \text{gr/lb}$, $h \approx 25.9\ \text{Btu/lb}$).

Note that the supply air has a higher humidity ratio than the outdoor air. Heating the outdoor air alone would never produce it — heating runs at constant $W$ and would give 140°F / φ = 7%. The supply state is a mixing point: roughly 20% recirculated exhaust and 80% fresh air, and only then heated. So 79,000 lb/h circulates through the kiln, but only about 63,000 lb/h leaves it and is replaced by fresh air.

Drying time:

$$t = \frac{6{,}600}{79{,}000 \cdot 0.0417} \approx 2\ \text{hours}$$

This is the theoretical (shortest possible) time, assuming the exhaust air leaves at exactly φ = 45% throughout. In reality, drying takes far longer — once the falling-rate phase sets in (see above), the exhaust air is no longer as humid and the driving force of the process decreases.

The heat input is calculated from the fresh air flow and the exhaust state — that is the only way the latent heat of the water carried away enters the balance:

$$Q = \dot{m}_{\text{f}}\,(h_{\text{ex}} - h_{\text{out}}) \approx 63{,}000 \cdot (101 - 25.9) \approx 4.7\ \text{MMBtu/h}$$

The specific energy is $e \approx 4{,}730{,}000 / 3{,}300 \approx 1{,}430\ \text{Btu/lb}$ of evaporated water — inside the usual range and safely above the latent heat of water. Had we counted only the heating of the supply air, $79{,}000\cdot(55.0-25.9) \approx 2.3\ \text{MMBtu/h}$, we would have arrived at about 700 Btu/lb, less than the evaporation alone costs — and that is impossible.

More recirculation shrinks the heat source but lengthens the charge in proportion:

Recirculation $r$	Supply air	Heat input $Q$	Time	Specific energy $e$
0%	61 gr/lb (φ 7%)	5.9 MMBtu/h	1.6 h	1,430 Btu/lb
20%	133 gr/lb (φ 15%)	4.7 MMBtu/h	2.0 h	1,430 Btu/lb
40%	210 gr/lb (φ 23%)	3.5 MMBtu/h	2.7 h	1,430 Btu/lb
60%	280 gr/lb (φ 31%)	2.4 MMBtu/h	4.0 h	1,430 Btu/lb
70%	315 gr/lb (φ 34%)	1.8 MMBtu/h	5.3 h	1,430 Btu/lb

The relative-humidity column shows why recirculation is indispensable: supply air at φ = 7% would dry the lumber fastest, but the surface would check. (Verify the specific values for your conditions on the PsychroView chart.)

Applications across industries

• Wood processing — kiln schedules at 120–195°F; in US practice the USDA Forest Products Laboratory’s Dry Kiln Operator’s Manual is the standard reference, while European mills assess dried lumber quality to EN 14298.
• Food industry — low temperatures (105–140°F) to preserve nutritional value, with precise humidity control.
• Paper industry — very high temperatures (210–300°F), continuous operation, the main energy consumer of the production.
• Ceramics and building materials — slow drying without thermal shocks, to avoid cracking.

Frequently asked questions

Why does the material temperature not rise in the first phase of drying? Because all the supplied energy goes into evaporating the free water. The temperature of the wet surface in the flowing air settles at approximately the wet-bulb temperature of the drying air — only after the free water is removed does the material start to heat up.

How much can recirculation save? The heater capacity falls in proportion to the vented fraction — with 60% recirculation, to roughly 40%. On its own, though, that saves no energy in drying the charge: the drying takes proportionally longer and the consumption per pound of evaporated water stays the same. That figure is set by the exhaust state alone.

Do drying lumber and grain differ? The principles are the same; the temperature ranges and the product’s sensitivity differ. Drying crops is covered in the article Psychrometrics in agriculture.