Evaporative cooling systems are indeed one of the most effective methods for cooling greenhouses in hot and dry regions. Accurate calculations for the components of such a system, including pads and fans, ensure optimal efficiency and maximize their benefits.
The references for these calculations, as you mentioned, include:
Chapter 4 of “Greenhouse Management” by Paul V. Nelson, translated by the Tehran Parks and Green Spaces Organization.
“Greenhouse Engineering” by Robert E. Aldrich.
The required tables have been directly sourced from these books.
table of content
Cooling system calculation
Several parameters are effective in determining the amount of exhaust air volume, including altitude above sea level, light intensity of the area, temperature difference at the beginning and end of the greenhouse, and the distance between the pad and the fan. By referring to the relevant tables for the area, we will have:Altitude above sea level of the area 1550 meters and referring to Table 4-1
Felev =1.2
Maximum light intensity of the area 53.8 Klux and referring to Table 4-2
FLight =1.0
Temperature difference between the initial and final 3.5 degrees Celsius and referring to Table 4-3
FTemp =1.12
Distance between the pad and the fan 48 meters and referring to Table 4-4
FVel =1.0
Fhouse = Felev.FLight.FTemp
Fhouse =1.2 x 1.0 x 1.12 = 1.344
This coefficient of FVel is the basis for our calculations. Fhouse is larger, so the air movement in a standard greenhouse is equal to 8 cubic feet per square foot of greenhouse floor area per minute.
The obtained number is corrected by the Fhouse coefficient.
The total amount of air movement for a greenhouse with an area of 3,072 square meters (8 openings, 8 meters wide and 48 meters long) will be:
3072.10/76.1/344.8=
355.404 =
604.187 =
The factor 10.76 converts square meters to square feet and the factor 1.7 converts cubic feet per minute to cubic meters per hour.
Number of fans
The 1.4 meter diameter fans are powered by a 1.1 kW motor. These fans have an exhaust capacity of 45,000 cubic meters per hour at a pressure of zero pascals (when there is no obstacle to the air suction). With the cellulose pad, this suction capacity is calculated at a pressure of 30 pascals. As can be seen in the table above, the air discharge capacity drops to 40,310 cubic meters per hour.
15 = 604.187 / 40.310 =
If 1.4 meter fans are used, we will need 15 fans, and for symmetry, 2 fans can be used in each opening, which will result in 16 fans. This number of fans will be installed in a length of 64 meters, so the distance between the fans will be 4 meters. The maximum allowable distance between the fans is 7.6 meters, and the distance between the fans is within the allowable limit.
Cellulose pad
The allowable air volume for a 10 cm cellulose pad is 250 CFM per square foot. Considering the suction power of the fans, for each fan with a 1 meter opening (with a capacity of 16,180 cubic meters per hour), we will need 3.5 square meters of 10 cm pad, and for a fan with a 1.4 meter opening (with a capacity of 40,310 cubic meters per hour), we will need 8.8 square meters of 10 cm pad.
132 square meters = 15.8/8
In this greenhouse, with 15 fans, we need 132 square meters of 10 cm pad. This amount of pad will be installed in a length of 64 meters, so the pad will be installed with a height of 2 meters and a length of 64 meters. (128 square meters will be installed in practice)
water pump
For each meter of length of a 10 cm cellulose pad, water must be transported upwards at a rate of 6.2 liters per minute (page 225 of the Greenhouse Management book). For longer lengths, two or more similar distribution systems are used. For a 64 m length, the pump capacity required will be 396.8 liters per minute.
The pump calculation for a 15 cm pad will be 9.3 liters per minute per meter length.
Store size
The water tank volume is 30.5 liters per square meter of 10 cm cellulose pad (page 229 of the Greenhouse Management book). A 4000 liter tank pad is sufficient for 128 square meters. The water tank volume is 40.7 liters per square meter of 15 cm cellulose pad.
electricity consumption
The fans operate with a 1.5 horsepower or 1.1 kW motor and their current consumption is 2.8 amps. Considering 16 fans, the power consumption will be 44.8 amps, which is added to the water pump consumption, so a minimum of 50 amps of power will be required for a 3072 square meter greenhouse cooling system.By referring to Table 1-1 of the Greenhouse Engineering book, we could have a good estimate of electricity consumption from the beginning. In this table, 50 amps are predicted for a greenhouse up to 2700 m2 and 66 amps for a greenhouse up to 3700 m2. This table can be viewed in its entirety. It should be noted that the largest energy cost in a greenhouse is for the cooling system.
Frame installation considerations
The aluminum pad frame consists of three main parts:
Bottom channel frame
Top frame
Top cover (installed after placing the pads)
The pad frame is 13 cm longer than the pad length to accommodate proper fitting. The ends of the bottom channels are sealed with galvanized sheets and silicon paste for waterproofing. The frame is installed level, and the slope is only considered in the outlet pipes.
Water management:
The water inlet is fitted with a float valve positioned at the mid-level of the tank to prevent overflow when the pump is turned off and water returns to the tank.
A submersible pump circulates water from the tank's bottom, and the water flow rate is controlled by the bypass valve.
The Off Bleed valve removes 1–2% of the circulating water to ensure fresh water inflow exceeds evaporation on the pads, preventing salt buildup.
Water distribution:
Distribution pipes are 4 (1¼ inches) and should not exceed 15 meters in length.
The pipes are drilled every 6 cm with 3 mm holes, positioned upward to eliminate the need for air bleeding and prevent clogging from suspended particles.
Additional setup:
Separate water valves should be provided for each frame in case of independent pad networks.
The water transfer system follows a spiderweb distribution, as linear setups may lead to uneven water distribution.
Evaporation rates: Water evaporates at a rate of 0.4 liters per minute per square meter of pad surface on a hot, dry day. For 128 m² of pads, the water consumption will be 51.2 liters per minute.
Electrical system considerations
When wiring the fans for the greenhouse, the following steps should be considered:
Phased Activation:
Initially, only half of the fans (odd rows) should be activated. If the desired temperature is not achieved, the remaining fans (even rows) can be brought into operation.
Delay Relays:
Due to the high inrush current of motor startups, all fans in a group should not start simultaneously. Use delay relays to sequentially activate the fans.
Pre-Startup Check:
Ensure the windows behind the pads are open before starting the fans to enable proper airflow.
Maintenance, service and installation recommendations
Best Practices for Managing Cooling Systems in Greenhouses:
Managing Water with High Mineral Content:
Replace the tank water at least weekly to avoid salt buildup in the system.
Fan and Pad Placement:
Install fans on the south and pads on the north side of the greenhouse to prevent shading inside and reduce rapid evaporation and salt deposits on the pads due to sunlight.
For east-west installations, place pads on the east side to maximize exposure to the west's sunlight.
Airflow Considerations:
Ensure crops inside the greenhouse do not obstruct the movement of cooling system air. Air should flow freely through the plants.
In multi-crop greenhouses, arrange plants based on their temperature sensitivity to account for temperature differences between the pad and fan sides.
Cooling Objective:
The primary aim of the cooling system is to maintain the plants' optimal temperature. The temperature below the growing beds and above the plants is less critical.
Install fans and pads at a height that ensures airflow passes directly through the plant level without moving beneath them.
Thank you for sharing these insightful details! If you have further points to add or would like assistance with related calculations or configurations, feel free to ask. — Prepared by Marsos Cam Greenhouse Industries, Hassan Farghani.
