Free area, not chamber volume, sets the flow
People size a drying chamber fan from the volume of the box and an air changes figure. That gets you a fan and tells you nothing about whether air reaches the middle of the stack. What matters is the free area between the trays: the length the air travels along, times the sum of the gaps.
For a 26 inch tray with fourteen gaps of 3 inches, that is 26 by 42 inches of open face, or 7.58 square feet. At 100 feet per minute across it, 758 CFM. The chamber volume never enters the calculation at all.
| Clear gap | Levels in 58 in of stack | Free area, 26 in trays | CFM at 100 FPM |
|---|---|---|---|
| 2 in | 19 | 6.86 sq ft | 686 |
| 3 in | 14 | 7.58 sq ft | 758 |
| 4 in | 11 | 7.94 sq ft | 794 |
| 6 in | 8 | 8.67 sq ft | 867 |
The interesting thing in that table is how little the CFM column moves. Tighter spacing gets you more levels, and more levels largely makes up for each gap being smaller, so the free area changes by about a quarter across the whole range. The tray count, meanwhile, goes from 8 to 19 — it more than doubles. The cost of tight spacing is not flow; it is that each individual gap is now narrow enough that any unevenness in the load blocks it, and a blocked gap gets no air at all.
The air runs out of room, and that is the real limit
Air at 60 degrees is saturated at 77.3 grains of moisture per pound. If it enters the racks at 70% relative humidity it is already carrying 53.8, so it has 23.5 grains of headroom before it can take nothing more.
How much it actually picks up crossing the stack is the moisture rate divided by the mass of dry air going past, and the mass is easy: 4.5 pounds of dry air per hour for each CFM. A quarter of a pound of water an hour is 1,750 grains an hour; across 758 CFM that is 3,411 pounds of air, so the pickup is 0.51 grains per pound and the air leaves at 70.7% instead of 70%. Nowhere near the headroom, so the airflow is comfortably in excess and the fan is not what is limiting the drying.
Halve the airflow twice and quadruple the moisture rate and you land in the other regime, where the air leaves the stack saturated. Then the trays at the downstream end are sitting in spent air and drying far more slowly than the ones at the front — which is the whole reason people rotate trays, and the reason rotating them is a workaround rather than a fix.
Drying is cooling, and it is not a small effect
Evaporating a pound of water absorbs around 1,060 BTU. A chamber losing a quarter of a pound an hour is therefore absorbing about 265 BTU an hour from somewhere, and that somewhere is the air and the load. In an insulated box with a small heater this is a large fraction of the heat input, and it is why a chamber can sit below its setpoint whenever the load is drying fast and drift up as the load slows down.
The same term appears as a load on any dehumidification equipment: the pints per day figure on this page is what a machine would have to remove if it were removing all of it, in the same units the dehumidifier sizing calculator uses.
What this page will not tell you
Any temperature, humidity, airflow or time for drying or curing anything edible. Those come from a tested written process — your state extension service publishes them, and a food safety professional is where a question about a specific product belongs. Nothing here is a judgement about whether a process is safe or whether what comes out is fit to eat, and no figure on this page should be read as one.
Two practical hazards worth stating without a procedure attached. A heated, humid, fan-driven box is an electrical appliance in a wet environment, and how it is wired, protected and earthed is licensed work. And a walk-in sized chamber is a confined space; air that has been recirculating over a load is not automatically air a person can breathe, and anything giving off gas needs to vent somewhere other than the room you are standing in.
Related
The fresh-air version of the flow question is the CFM and air changes calculator, and the duct that carries the air is the duct static pressure calculator. For the moisture load in pints and what a machine does with it, see dehumidifier sizing and the dehumidifier run time calculator. Drying a wet building rather than a load on racks is the water damage drying calculator. For the shell around the chamber, the cellar insulation take-off lists the materials.
Questions people ask
How far apart should drying racks be?
The calculator shows the trade rather than naming a number, because the right answer depends entirely on how thick the load sits on the tray. What the arithmetic shows is that free area moves only from 6.9 to 8.7 square feet between a 2 inch gap and a 6 inch one, while the tray count falls from 19 to 8 — you get many more levels with tighter spacing and each gap is only slightly smaller. What changes is risk: a narrow gap is easily blocked by an uneven load, and once it is blocked that level gets no air at all. Measure the gap to the top of the load, not to the tray.
How many CFM does a drying chamber need?
Take the free area between the trays in square feet and multiply by the face velocity you want in feet per minute. For 26 inch trays with fourteen 3 inch gaps that is 7.6 square feet, so 100 FPM is 758 CFM. Sizing from chamber volume and air changes instead will give you a number, but it says nothing about whether air reaches the middle of the stack, which is the thing that actually decides how evenly the load dries.
What face velocity should I use?
That comes from your process, not from this page. What the calculator can tell you is the consequence of the number you pick: the CFM it implies, and how much moisture the air picks up crossing the stack. If that pickup is small compared with the saturation headroom, airflow is not your limiting factor and raising it further buys little. If the air leaves the racks saturated, airflow is the limit and everything downstream is drying slowly.
Why does my chamber run below its setpoint while the load is wet?
Evaporation absorbs about 1,060 BTU for every pound of water that leaves. A load shedding a quarter pound an hour is pulling around 265 BTU an hour out of the air, which in a small insulated box is often more than the heater is putting in. As the load dries the rate falls and the temperature climbs. It is normal, it is calculable, and the latent figure on this page is the size of it.
Can this tell me how long to dry something, or whether it is safe?
No, and it will not. This page does airflow and moisture arithmetic on numbers you supply, including the weight loss and the run length, both of which come from your own measurements. Times, temperatures and humidities for anything edible come from a tested recipe or a food safety authority — your state extension service is the usual place to find a written, tested process — and whether a finished product is fit to eat is a question for a food safety professional.