June 2019 - 36

GREENHOUSE EQUIPMENT
Airing It Out
Naturally ventilating your greenhouses can help control energy costs and produce healthy plants.
BY NEIL MORAN
L
ike any greenhouse business, you probably have concerns about
energy costs, best ventilation practices and how to best invest your
hard-earned money. It is understandable that you want to make the
best decisions that will get you off on the right foot. To choose the right
type of ventilation for your greenhouse, it's important to take an in-depth
look at the features and cost-effectiveness of each.
MECHANICAL VENTILATION
In mechanical ventilation, fans are installed in a greenhouse to exhaust
the rising hot air. This creates a vacuum that draws cooler air through
louvers on the end of the greenhouse. When properly designed, this type
of ventilation can maintain ideal internal greenhouse temperatures even
while operating in a variety of locations and weather conditions.
There will need to be multiple greenhouse ventilation fans installed to
provide ventilation at various rates. Ventilation fan motors also should
have multiple speeds to allow for user control of the ventilation rate.
CURTAIN VENTILATION
For most growers in the temperate zones, natural ventilation is the best
choice for climate control and cooling in a greenhouse. Natural ventilation,
or curtain ventilation, uses a series of roof and sidewall vents. As the
temperature increases within the greenhouse, hot air rises and escapes
through the roof or sidewall vents. This creates a vacuum that draws
cooler air into the greenhouse through the sidewall vents located closer to
the ground.
Curtain ventilation systems allow for air movement across the width of
your greenhouse. Since the width is the shortest dimension, the new air
will stay cooler than air moving along the entire length of a greenhouse,
which is common in mechanical systems.
Since these systems don't require fans, they are much more energy
efficient, helping to reduce operating costs. They're also easy to automate
with climate controls to increase crop yields and labor efficiency.
Natural ventilation offers many advantages over the traditional fan
and louver cooling, a big one of course, being the energy savings. " It
depends on the use, but the trend is toward natural ventilation, " says
John W. Bartok, Jr., a consultant who does energy audits for green
industry businesses and works with the USDA on energy issues. " Even
high tunnels, which generally never had vents - some are now putting
them in to allow for natural ventilation. "
Bartok attributes this trend to the increasing costs for electricity, the
costs of fans and fan installation, and the " positive ventilation " that is
achieved with natural ventilation.
36 JUNE 2019 GPNMAG.COM
Automated roll up curtain system.
Natural ventilation accounts for 80% or more of the ventilation in a
greenhouse, according to Bartok, something that is costly to achieve with
fans alone. " You need to run a lot of exhaust fans to recoup the amount
of air that would flow through natural ventilation, " says Dave Stoltzfus,
president at Advancing Alternatives. " If you have three, 36-foot fans
in a 30x96-foot greenhouse you would still fall short of providing the
recommended air flow of 40,000 cu.ft/min. Natural ventilation will easily
provide that amount of airflow without having to turn on a fan. "
Stoltzfus adds, " If you open up the sidewalls 6 feet on a 30x96-foot
greenhouse and the air is moving at 2 mph - and it is uncommon that it
would be less - natural ventilation will move the air through fairly quickly. "
Stoltzfus also conveyed that not all growers take into account that
fans must be kept running during the heat of the summer - when the
greenhouse is often empty - just to keep the greenhouse poly from
deteriorating. Natural ventilation, however, will eliminate the need to run
fans during the summer months, further reducing energy costs.
In addition, Bartok says fan cooling can result in a temperature
difference of up to 8 degrees from one end of the greenhouse to the other,
possibly resulting in uneven plant growth. Properly configured natural
ventilation systems offer more even temperatures throughout a greenhouse

http://www.GPNMAG.COM

June 2019

Table of Contents for the Digital Edition of June 2019