Group Title: Circular
Title: Fan and pad greenhouse evaporative cooling systems
Full Citation
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 Material Information
Title: Fan and pad greenhouse evaporative cooling systems
Series Title: Circular
Physical Description: 7 p. : ill. ; 28 cm.
Language: English
Creator: Bucklin, R. A
Henley, Richard W
McConnell, Dennis Brooks, 1938-
Florida Cooperative Extension Service
Publisher: Florida Cooperative Extension Service, Institute of Food and Agricultural Sciences, University of Florida
Place of Publication: Gainesville
Publication Date: 1993
Subject: Greenhouses -- Cooling -- Florida   ( lcsh )
Greenhouses -- Climate -- Florida   ( lcsh )
Genre: government publication (state, provincial, terriorial, dependent)   ( marcgt )
non-fiction   ( marcgt )
Statement of Responsibility: R.A. Bucklin, R.W. Henley and D.B. McConnell.
General Note: Title from caption.
General Note: "December 1993."
 Record Information
Bibliographic ID: UF00008562
Volume ID: VID00001
Source Institution: University of Florida
Holding Location: University of Florida
Rights Management: All rights reserved by the source institution and holding location.
Resource Identifier: ltqf - AAA6826
ltuf - AKB7907
oclc - 31070208
alephbibnum - 001941722

Full Text



Circular 1135
December 1993

Institute of Food and Agricultural Sciences

Fan and Pad Greenhouse Evaporative Cooling Systems1

R. A. Bucklin, R. W. Henley and D. B. McConnell2

Without cooling systems, temperatures in Florida
greenhouses during the summer can easily exceed
100 F. Temperatures this high reduce the quality of
many crops and will greatly reduce worker
productivity. Almost all greenhouses cooling systems
use some form of evaporative cooling to operate. Air
conditioning or refrigeration systems can be used, but
their installation and operating costs are usually too
high to justify their use.


Evaporative cooling is a process that reduces the
temperature of air by the evaporation of water into
the airstream. As water is evaporated, energy is lost
from the air reducing its temperature. Two
temperatures are important when dealing with
evaporative cooling systems. The first is the dry bulb
temperature. The dry bulb temperature of the air is
the temperature that we usually think of as air
temperature. It is the temperature measured by a
regular thermometer exposed to the airstream. A
second air temperature is important in evaporative
cooling systems. This temperature is referred to as
the wet bulb temperature. The wet bulb temperature
is the lowest temperature that can be reached by the
evaporation of water. The wet bulb temperature is
the temperature you feel when your skin is wet and is
exposed to moving air. The dry and wet bulb
temperatures can be used to calculate the relative
humidity. The equipment used to evaporate water,
move cooled air through the greenhouse, and exhaust

warm air from the greenhouse comprises the fan and
pad cooling system.

It is the wet bulb temperature and not the relative
humidity that determines to what temperature air can
be cooled by evaporation of water. During the heat
of the afternoon when the dry bulb temperature is
normally at its peak, the difference between the dry
bulb and wet bulb temperature is the greatest. Thus
the greatest potential for cooling is obtained during
the heat of the day when it is needed most. Wet bulb
temperatures can be determined by checking with
your local weather station or by investing in an
aspirated or sling psychrometer. These psychrometers
consist of two thermometers. The end of one
thermometer is covered by a wetted wick. The other
thermometer is a conventional thermometer. These
two thermometers are exposed to a moving airstream
so that they measure the wet and dry bulb
temperatures. The difference between sling
psychrometers and aspirated psychrometers is the way
the airstream is provided. A sling psychrometer is
mounted on a swiveled handle and whirled rapidly,
while an aspirated psychrometer uses a small fan to
provide air movement.

Weather data collected by the weather bureau for
many years indicates that afternoon wet bulbs in
Florida are about 79-80 F during hot weather
conditions. The most critical time to check wet bulb
temperatures is in the afternoon when solar radiation
and outside temperatures are highest. With an

1. This document is Circular 1135, Florida Cooperative Extension Service, Institute of Food and Agricultural Sciences, University of Florida.
Publication date: December 1993.
2. R.A. Bucklin, associate professor, Agricultural Engineering Department; R.W. Henley, professor, Central Florida Research and Education
Center, Apopka, FL; D.B. McConnell, professor, Environmental Horticulture Department, Cooperative Extension Service, Institute of Food
and Agricultural Sciences, University of Florida, Gainesville FL 32611.
The Institute of Food and Agricultural Sciences is an equal opportunity/affirmative action employer authorized to provide research,
educational information and other services only to individuals and institutions that function without regard to race, color, sex, age, handicap,
or national origin. For information on obtaining other extension publications, contact your county Cooperative Extension Service office.
Florida Cooperative Extension Service / Institute of Food and Agricultural Sciences / University of Florida / John T. Woeste, Dean


Fan and Pad Greenhouse Evaporative Cooling Systems

efficient, well managed system, you should be able to
reduce the temperature of the air entering the house
to within 3-4 F of the wet bulb temperature. Re-
member, this will not be the temperature in all areas
of the house. As the air moves across the house to
the exhaust fans, it will pick up solar heat so that the
exhausted air will likely be 7-8 F higher than the
entering air. In a poorly managed system, the
exhausted air could have a much greater temperature

The basis of any evaporative cooling system is the
evaporation of water into an airstream. The most
common way of accomplishing this is the fan and pad
system shown in Figure 1.

Exhaust FansWith Boxes, Shutters and Guards

Fans Spaced at 0 to 80'

iITI. 11111

Prevailing Wind

Figure 1. Typical pad-fan greenhouse arrangement.

In recent years, high pressure fog systems have
started seeing use in greenhouses. These fog systems
can be designed and operated to maintain more
uniform temperatures and humidities in greenhouses
than are possible with fan and pad systems. Fog
systems are more expensive than fan and pad systems
but when uniform temperatures and high humidity
levels are important they can be the best method of
evaporative cooling.


Fan and pad systems consist of exhaust fans at
one end of the greenhouse and a pump circulating
water through and over a porous pad (Figure 2)
installed at the opposite end of the greenhouse.

If all vents and doors are closed when the fans
operate, air is pulled through the wetted pads and

Page 2




Figure 2. Evaporative cooling pad. The sump should be
large enough to hold all run-off when the pump is turned

water evaporates. As each gallon of water is
evaporated, 8,100 BTUs of heat energy are absorbed
from the air by the water in the change from liquid to
vapor. Removing energy from the air lowers the
temperature of the air being introduced into the

The air will be at its lowest temperature
immediately after passing through the pads. As the
air moves across the house to the fans, the air picks
up heat from plants and soil and the temperature of
the air gradually increases. A temperature gradient
across the greenhouse results, with the pad side being
coolest and the fan side warmest.


The temperature gradient across the greenhouse
is hard to predict because many variables affect the
gradient. Some of these are bench arrangements,
physical obstructions to the movement of air across
the house, percentage of floor area covered by plants,
or whether the floor is bare soil or covered with
concrete. The configuration of the roof can also have
an influence on temperature gradient. Experience
has shown that air may heat up as rapidly as 1*F for
every 10' of movement on sunny summer days. The
slower the air movement, the faster the air heats up,
and the greater the gradient.

The temperature gradient across the house at
plant level is most important. In most systems, the
air tends to diverge upward at an angle of about 7
above horizontal, or roughly 1' in 8'. The upper layer
of cooled air rises toward the peak of the building
above the crop zone and thus does little cooling of
the plants. In a cross flow arrangement of gutter

Fan and Pad Greenhouse Evaporative Cooling Systems

connected houses, the gutters serve as baffles and
tend to keep the cool air at crop level. In
longitudinal flow arrangements where the air flows
lengthwise through the house, triangular shaped
baffles need to be placed extending from the roof,
tapering down to just above the top of the crop level.
The baffles should be transparent and spaced about
30' apart. These baffles should be in a fixed position
(Figure 3).

-- - - -- - FANS



-- -- -- -- ----- A- FANS

Figure 3. Pad-fan arrangements in greenhouses.

Where plants are grown on raised benches, baffles
about two-thirds of the way down the sides of the
bench will force most of the cooling air to crop level
for more effective cooling.


A well designed, properly installed and operated
evaporative cooling system may have an operating
efficiency of up to 85% (Figure 4). The difference
between dry bulb temperature and wet bulb
temperature is referred to as the wet bulb depression.
By using a psychrometric chart (Figure 5) with the
outdoor temperature and relative humidity, you can
calculate the wet bulb temperature that, theoretically,
would be the temperature of the entering air.

If the efficiency of your system is 85%, then the
pntir;nn air tpmnprnatfrp wnunld hP thf wP.t hulh tenm-
r- --r -- ---- -- -- -r- ---
Figure 5 shows what happens to air with a dry bulb
temperature of 95 F and with a wet bulb temperature
of 80 F that is cooled using an evaporative cooling
pad. If the pad is 85% efficient, the air is cooled to
point A or 82*F. If the pad is 70% efficient, the air
is cooled to point B or 84.5F, and if the pad is 50%
efficient, the air is cooled to point C or 87.5*F.

Page 3

70 t---I.---1I I t1 I I1
00 60 70 60 g0 100

Figure 4. Cooling potential c 85 percent efficient
evaporative cooling systems.


40 60 80 1 10

Figure 5. Psychrometic chart showing the effect on
operating efficiency of entering I air temperatures.


House Cc istruction

Managing the house o make sure that it is as
tight as possible, and mak ng sure that all doors and
other openings are always cept closed except when in
iinue will have a dramatic effect on the success or
:- ooling system. It is very
important to keep the building as tight as possible so
entering air will be forced to come through the pads.
If a door is left open, a vent is unclosed or uncovered,
or if excessive cracks are not caulked, it will be easier
for the air to take these routes into the building
rather than through the cooling pad. An ineffective
system will result.

Fan and Pad Greenhouse Evaporative Cooling Systems

House Location and Orientation

Orientation of the greenhouse in relation to other
buildings or structures and in relation to prevailing
summer winds has an influence on efficiency of
operation and may affect fan arrangements or
whether the pad fan will be located on the side or end

Type of Cooling Pad

The most widely used type of pad material is
corrugated cellulose that has been impregnated with
wetting agents and with insoluble salts to help resist
rot. These pads are expensive but when properly
maintained do an excellent job of cooling air and if
properly maintained, should have a lifetime of ten

Aspen pads have seen wide use in the past and
some are still in service. However, under Florida
conditions the life of an aspen pad is usually short.
Aspen pads are very susceptible to algae infestation
that leads to rotting and compaction. This makes it
difficult to maintain an efficiently operating system
without frequent and costly pad replacements.

Other pad materials are also on the market, but
none has seen wide acceptance. Among these are
pads fabricated from aluminum and from plastic
fibers. Both these pads types are expensive and have
not shown advantages over corrugated cellulose.
However, an operator planning to replace an old pad
system or install a new one should check out
completely all the pad materials available. Then
compare costs, life expectancy claims, cooling
efficiencies, probability of maintenance problems
before selecting the one that appears best for the

Cooling Pad Area

The amount of pad area needed depends upon
several factors, including the type of pad material
used. The pads should be continuous along the entire
wall. If aspen pads are used, it is recommended that
one square foot of pad be provided for each 140 cu.
feet per minute (CFM) of air moved by the fans. The
fan capacity should be based on total CFM delivered
at 1/8" static water pressure. Cellulose pads can be
used with airflows of up to 230 cu. feet per minute
per square foot of pad. With the higher airflow rates
of cellulose pads, fewer square feet of pad area are

Page 4

Water Flow Rate

You must have adequate pad surface area and an
adequate water supply and distribution system. The
amount of water needed will vary with the type of
system used, but normally, to assure complete pad
surface wetting, about 1/3 gal of water per foot of pad
length is recirculated. A valve should be placed in the
line from the pump so the flow to the distribution
pipe can be adjusted. You do not want a sheet of
water flowing down the pad surface. This would
increase resistance to air flow and cause transfer of
free water into the house. You do want the pad
surfaces covered with a water film. If in doubt about
the correct quantity of water flow, check with the pad
manufacturer. The water collected by the bottom
gutter is returned to a sump from which the water is
pumped to the upper distribution pipe or gutter. For
houses more than 75' in length, it is usually more
efficient to locate the sump near the center of the
house. The sump should have a capacity of 1 to 11/4
gal for each linear foot of pad in order to hold the
water that drains back to the sump when the system
is stopped.

Problems With Cooling Pads

Evaporative cooling pads have severe problems in
Florida. They are often subjected to many un-
desirable factors, such as clogging due to impurities in
the water, algae and decay. If the pad material is
clogged or decomposed its ability to function as
designed is impaired. Air exhausted by the fans will
enter the building at the points) of least resistance.
If a pad area is totally or partially clogged, very little
if any air will pass through that portion of the pad. If
the pad has holes, the air will take the path of least
resistance. This means less contact between air and
water and much less cooling. When a pad has de-
cayed, the only alternative is to install a new pad.

Airflow Through Cooling Pads

The required face velocity of the air will depend
upon the pad material. Follow manufacturer's
suggestions. This velocity will determine the number
of square feet of pad area needed for a house of a
given configuration. Regardless of the type of pad
material used, the fans should have the capacity to
provide a minimum of one air change per minute in
the greenhouse. Have automatic shutters on the fans
so there will not be back drafts when a fan is not
operating (Figure 6).

Fan and Pad Greenhouse Evaporative Cooling Systems

Construct the pad so that all air entering the
house will have to pass through the pad. Provide a
method of closing off the pads during the winter when
heating, not cooling, is required.

Figure 6. Ventilation and cooling fans should be equipped
with anti-backdraft shutters.


Fans should have the capacity to provide at least
one air change per minute. Heat enters the
greenhouse through the greenhouse covering and
causes the air temperature to rise. Solar heat loads
on greenhouses will vary according to latitude
location, as well as time of year and day. For
example, in central Florida it will vary from a
maximum direct normal solar irradiation in BTU per
hour per sq. foot of about 280 in June to 300 in
September (Table 1).

Table 1. Maximum direct normal solar irradiation.

Maximum direct solar Irradiation in
BTUs per hour per square foot

Date 240N Latitude 32"N Latitude

June 21 280 280
July 21 279 278
August 21 285 283
September 21 301 296

Location of Fans and Cooling Pads

The best distance between the pads and exhaust
fans is influenced by optimum dimensions of the
house from an efficiency, functional and operational
standpoint and the effective tolerance of plants
produced in the house to temperature differences.
The greater the crop's tolerance to temperature
differences, the greater the distance between pad and
fans can be. It is not practical to separate the pad

Page 5

and exhaust fans by more than 200'. A distance of
150' or less is preferred.

Location of pads and fans will be influenced by
several factors. Keep in mind:

" When possible, locate pads on the prevailing
summer wind side and locate the fans on the
downwind side of the greenhouse. Should the
pads be protected by another house within 25',
the wind effect is negligible and can be ignored.

If it is necessary to face fans into the prevailing
winds, increase fan CFM capacity 10 to 15% and
correspondingly increase fan motor horse power
and add shutters or back draft dampers.

The exhaust fans should not discharge toward the
pad of another house unless the houses are
separated by at least 50'.

When fans from two adjacent houses close to
each other exhaust into a common area between
the houses, they should be offset from each other
to avoid the air from one blowing directly against
the other (Figure 7). Roof mounted fans should
be used if fans do not have at least 1.5 fan
diameter clearance between their discharge
opening and the nearest obstruction.

The maximum practical distance from pad to fan
should never exceed 200'. Distances of 150' or
less are more effective to reduce the temperature
gradient. For most houses, about one foot of pad
height is required for every 20' of pad to fan

In very long houses, it is more efficient to locate
a pad at each end with the exhaust fans at the
center of the house using side wall or roof
mounted fans. Cooled air then flows in from
each end and is exhausted at the midpoint of the
house. All fans should be equipped with auto-
matic shutters for weather protection and to pre-
vent backdrafts when fans are not in use.

Special motorized roof housings are used for
mounting fans on the roof.

The fans should be properly screened and
guarded to safeguard personnel from coming in
contact with any moving parts (fan blades, pulleys
and belts).

A correctly designed pad system is essential to
achieve maximum cooling performance. It must

Fan and Pad Greenhouse Evaporative Cooling Systems

Figure 7. Typical greenhouse range layout.

be a continuous section along the entire side or
end of the greenhouse, be the correct size and
thickness. A blank space in the pad, such as a
doorway, will cause a hot spot through the house
for a distance of 6 to 8 times the width of the
blank. Exhaust fans should not be spaced more
than 20-30' apart (Figure 1).

Pads may be built inside the house or in the walls,
or they may be built outside the house (Figure 8).


Watch the pad condition carefully. If you notice
bare spots or thin areas in the pad, you can be sure
that much of the air entering the house is through
these areas, which will result in hot spots in the
house. To operate efficiently, the pad, pump system,
fan system and control systems must be designed to
operate as a unit. They must be checked frequently
to be sure all parts are functioning properly. Manage
the house operations so that doors are never left
open and are opened only when necessary to move
people or plants and equipment in and out. An open
door can reduce the effectiveness of a pad cooling
system significantly. The best house equipped with the
best possible equipment and sophisticated controls
can be a big loser without good management.


The evaporative pad cooling system must have
adequate controls for the operator to be able to
adjust the house environment to provide maximum
growing conditions for the plants and a comfortable
environment for workers. Thermostats are usually
used to turn fans and pumps on and off, as required
to meet changes in outdoor climate conditions and

maintain more uniform greenhouse temperatures with
lower operating costs. Thermostats should be
checked each spring and fall against an accurate
thermometer to insure proper operation.

A humidistat can be used to control pumps and
fans of the cooling pad system to help prevent
excessive greenhouse humidity. However, humidistats
are much less reliable than thermostats. If it is
desired to use a humidistat to control humidity, it is
recommended that the humidistat be checked at least
weekly to make sure that it stays in calibration.
Humidistats should be checked against a
psychrometer. Thermostats and humidistats must be
located at plant level to function properly and should
never be located on an outside wall.

Exhaust fans should be uniformly spaced not
more than 20 to 30' apart. The fans may be wired so
that the thermostats will turn on alternate banks of
fans in sequence as the temperature demands. Some
greenhouses are equipped with 2-speed fans which
should be controlled with 2-stage thermostats. This
arrangement will permit the first stage to turn the fan
on low speed and off as required while the second
stage will run the fan on high speed according to
demand. It is important that, during the time of year
a thermostat controls the heating system, the cooling
thermostat which controls the first stage fans should
be set 5-10OF above the setting of the heating
thermostat to avoid having the heating and cooling
systems on at the same time.

If evaporative cooling water pumps are controlled
by humidistats and thermostats, they should be wired
in series. This will help maintain more uniform
temperatures and avoid excessive humidities. It will
also help conserve power and water. In all cases,
however, a thermostat should be used as the main

Page 6

Fan and Pad Greenhouse Evaporative Cooling Systems

rAT ~ I~ G II I j

Figure 8. Typical section showing pad located outside building.

pump control. The thermostat should be set to stop
the pump before all the fans go off so that the pad
can dry out.

Each thermostat and humidistat should have a
manual control switch wired in parallel with it so that
manual control can be used when desired. A safety
disconnect switch should be located near each fan and
pump. All controls and instruments including
thermostats, humidistats and thermometers should be
shielded from the direct rays of the sun to avoid being
influenced by solar radiation and to provide more
correct readings and control settings. They should
also be mounted so that air can circulate freely
around the sensing elements and be located where
they represent the average greenhouse condition at
plant level. Aspirated thermostats at plant level are
the best choice. Do not locate near heating lines or
near an air inlet opening. This will distort the
readings from the true environmental conditions for
plants in the greenhouse.

Computers and Microcontrols

Thermostats and humidistats are reliable and
inexpensive, but are limited to simply turning pieces
of equipment on or off in response to a change in
temperature or relative humidity. Simple on-off
controls cannot regulate environmental conditions
exactly because they cannot sense how far the
temperature or relative humidity is from the set point,
or how rapidly the temperature or humidity is
changing. Computers and microcontrollers can use
software or hardwired circuits that incorporate logic
to make decisions about the exact amount of heat or
airflow required to produce desired environmental
conditions. The higher degree of control provided by
these devices results in more constant conditions that

provide a better environment for crop growth.

Computers and microcontrollers are rapidly
decreasing in cost at the same time that they are
increasing in reliability and sophistication. They are
now important tools that growers can use to increase
crop quality and increase profits.



Page 7


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