for agaricus farms, yeah, wood shelves are relics of bygone eras. newer farms
who have financial backing are using the dutch shelving systems. all of the exotics
farms i know are switching over to treated and conduit. it's less expensive and
treated, very unexpectedly, outlasts metal.
if you're pulling more air out than you're putting in, it creates negative pressure.
passive intake = negative pressure
the passive intake model is considered outdated because it lacks the precision
afforded to humidity conditioned air intake. most folks now are using the positive
pressure model, and putting a misting nozzle or ultrasonic fogger on the inside of
the fan, it makes a HUGE difference when the temperature is cold and humidity is
close to 0. for example, if it's 30 degrees F outside, and 100% RH at 30F, that air
is warmed to 60F by coming it into your space, the air will be in the lower 30%s RH
once it's inside. if you bring that air in through ducting and have a mid pressure
misting nozzle angled to disperse with the air flow, you can achieve 80%+, given
enough turbidity to facilitate evaporation.
here's a chart
and a graph
to illustrate the relation of temperature and humidity. both are screencaps from the
engineering toolbox website.
the air filtered intakes are common on smaller scale agaricus farms (which most people
inexperienced with agaricus farms see as huge because they produce thousands of
pounds when the real scaled people produce millions) with huge amounts of buffer air
space, but the bigger guys (and the ones who will be more successful and eventually
buy out the smaller, less organized farms) don't leave it up to the environment and
maximize their space.