July 2024 - 30

CEA
the leaf is always 100% because of
continuous transpiration. Therefore,
the LST is the sole variable that
controls the VPD.
LSTs are a function of the lighting
intensity, air temperature and
air velocity surrounding the leaf.
Artificial lighting used to promote
photosynthesis gets carefully
distributed to ensure uniform
exposure on the canopy. To be
effective, there should be a uniform
distribution of CV.
CV involves more than just blowing
air at the foliage. Surprisingly, the
most challenging engineering task
to control VPD is designing a CV
system that uniformly frees heat and
humidity from the foliage. The design
of this system should be part of the
early design phase.
Collaboration is essential between
the HVAC engineer, cultivator, and any
third party involved with the design
of the structure that houses the plant
lighting and irrigation system.
The current protocol for CV entails
design by a cultivator, who is usually
inexperienced in airflow dynamics.
This results in a haphazard array
of ducts weaved throughout the
structure, or fans blowing air at the
canopy, while leaving sufficient room
for maintenance.
Ducts or fans must not block light
waves or otherwise cast shadows
that interfere with photosynthesis.
Plant architecture teaches that
water vapor discharges from the
stomata on the leaf surface while
radiant light waves heat the leaf
surface. Temperature and water
vapor hug the leaf's surface,
forming a thin boundary layer.
To manage the VPD, the ventilation
Figure 1. Airflow spreads as it moves
from its source, covering a wider area
with diminishing velocity. Data courtesy
of Walter Stark Consulting.
system must continuously provide
turbulent air to disturb this layer
by sending heat and humidity into
the cultivation room for processing
through the HVAC system. This
requires a CV system that provides
sufficient and uniform air turbulence
inside the canopy.
There is no definitive advice about
the air velocity needed in the canopy.
It may depend on the type of plants
and environmental conditions.
However, current practices propose
an air velocity of 40-100 f/m, which
translates to a minimum of 40-100
f3
/m/f2
of canopy.
In contrast, a light outdoor
breeze has an air velocity of 300600
f/m. Regardless of the actual
requirement, the air volume for CV is
likely to be significantly greater than
that needed for HVAC.
Ventilation air is separate from
the air needed for temperature and
humidity control. The CV system
provides sufficient air velocity
throughout the canopy and keeps
the LST close to room temperature,
while minimizing excess ventilation.
Light waves that serve
photosynthesis also emit radiant heat
to the leaf surfaces. CV cools the
leaf surfaces, which prevents heat
buildup and lowers the LVP and VPD.
Canopy air volume and velocity
govern the ability of ventilation air
to penetrate the foliage and cool
the leaf surfaces. Too much velocity
is better than too little. However,
the ventilation system cost and
efficiency benefits from velocity that
is " just right. " The designer finds
the happy medium through trial and
error, mixed with experience and
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July 2024

Table of Contents for the Digital Edition of July 2024
