Views: 0 Author: Site Editor Publish Time: 2026-05-28 Origin: Site
In extreme summer heat, uncontrolled greenhouse temperatures lead to plant stress, yield loss, and operational failure. However, a space feeling "too hot for humans" does not necessarily mean it is fatal for crops. Plants primarily suffer from stagnant air, sudden temperature spikes, and root-zone baking. High-capacity cooling is necessary, but selecting the wrong system based on local climate data wastes capital and increases disease risk. For many commercial and serious hobbyist growers, greenhouse evaporative cooling offers high ROI, but its effectiveness is strictly governed by regional humidity levels and proper implementation.
Performance Ceiling: Evaporative cooling efficiency is limited by the "Wet Bulb Depression" (WBD). It performs best when relative humidity remains below 60%.
System Design: A cooling pad and exhaust fan greenhouse system can reduce greenhouse temperature effectively in dry and moderately humid climates.
Layout Requirement: Cooling pads and exhaust fans should be installed on opposite sides to pull cooled air across the crop area.
System Alternatives: In high-humidity zones where evaporative systems fail, growers may need closed-loop air conditioning, stronger ventilation, or passive shading.
Micro-climate Risks: High-pressure fogging creates fine droplets that evaporate quickly, while low-pressure misting may wet foliage and increase fungal disease risk.
To evaluate evaporative cooling, you need to understand Wet Bulb Depression, also called WBD. WBD is the difference between dry-bulb temperature and wet-bulb temperature. Dry-bulb temperature comes from a standard thermometer. Wet-bulb temperature shows how cool the air can become through water evaporation.
The larger the WBD, the stronger the cooling potential. When the air is dry, water evaporates quickly and removes heat from the air. This is why evaporative cooling performs better in hot and dry climates.
Evaporative cooling works best when relative humidity stays below 60%. Above this level, the air already contains too much moisture. Water evaporation slows down, and the cooling effect becomes weaker.
In hot and highly humid climates, evaporative systems may act more like humidifiers than coolers. They add moisture but do not reduce temperature enough. This can increase mold risk, reduce plant transpiration, and make heat stress worse.
Before choosing evaporative cooling equipment, check local summer climate data. Focus on the hottest part of the day.
Identify peak heat hours: Check the hottest daily window, usually between 1:00 PM and 4:00 PM.
Review historical humidity: Look at humidity data during the hottest summer months.
Calculate average WBD: Subtract wet-bulb temperature from dry-bulb temperature.
Make a data-driven decision: If peak-hour humidity often exceeds 60%, evaporative cooling may underperform.
A cooling pad and exhaust fan greenhouse system uses negative pressure ventilation and water evaporation to cool the greenhouse.
Exhaust fans are installed on one end wall of the greenhouse. Water-saturated cooling pads are installed on the opposite side. When the exhaust fans run, they pull hot outside air through the wet cooling pads. Water evaporates from the pad surface and absorbs heat from the incoming air. The cooled air then moves across the crop canopy and exits through the exhaust fans.
This design is simple, efficient, and widely used in commercial greenhouses. It is especially suitable for hot and dry regions where water evaporation can provide a strong cooling effect.
In suitable climates, a cooling pad and exhaust fan system can lower greenhouse temperatures significantly compared with outside air. The exact cooling result depends on outdoor temperature, relative humidity, cooling pad thickness, pad surface area, fan capacity, and greenhouse length.
The system performs best when the air is hot and dry. When humidity is high, the cooling effect becomes weaker because less water can evaporate into the air.
A cooling pad and exhaust fan greenhouse system can create a temperature gradient. The air is coolest near the cooling pad wall. As it travels through the greenhouse, it absorbs solar heat, crop heat, and ground heat. By the time the air reaches the exhaust fan end, the temperature may rise again.
To reduce this problem, the system should be designed with proper fan capacity, suitable cooling pad area, and good internal circulation. Horizontal airflow fans can also help mix the air and reduce hot spots inside the greenhouse.
The cost of a cooling pad and exhaust fan greenhouse system depends on greenhouse size, fan quantity, cooling pad area, water pump capacity, controller type, and installation requirements.
Maintenance is also important. Cooling pads are wet for long periods, so algae, dust, and mineral deposits can block the pad flutes. Once the pad is clogged, airflow drops and cooling efficiency becomes poor.
Best Practice: Use clean water and flush the water circulation system regularly to reduce mineral buildup.
Best Practice: Check the cooling pad surface, water distribution pipe, pump, and water tank during the cooling season.
Common Mistake: Letting pads stay wet overnight for long periods. Pads should be allowed to dry when cooling is not required.
Growers often use the words "fogging" and "misting" in the same way. In greenhouse cooling, they are different. The key difference is water droplet size.
High-pressure fogging systems create very fine droplets. These droplets can remain suspended in the air and evaporate quickly. Because they evaporate before falling onto the crop, they can cool the air without wetting leaves.
This rapid evaporation provides uniform greenhouse temperature control. Fogging is often used in propagation areas, seedling zones, and greenhouses that require stable humidity and temperature control.
Low-pressure misting systems create larger droplets. These droplets may not evaporate quickly enough. They can fall onto leaves, benches, and floors.
Wet foliage can increase the risk of fungal diseases such as powdery mildew and botrytis. Wet floors can also create safety risks for workers. For crop cooling, misting should be selected carefully.
Feature | High-Pressure Fogging | Low-Pressure Misting |
|---|---|---|
Droplet Size | Very fine droplets | Larger droplets |
Evaporation Rate | Fast evaporation | Slower evaporation |
Cooling Effect | More uniform cooling | Localized cooling |
Foliage Impact | Helps keep leaves dry | May wet plant leaves |
Best Application | Commercial cooling and propagation | Limited cooling or outdoor use |
Before investing in active cooling, reduce the greenhouse heat load first. Passive cooling lowers the pressure on mechanical systems and improves overall efficiency.
In extreme heat, airflow is essential. Active ventilation systems should be sized to achieve one full air exchange per minute. At minimum, the system should replace the greenhouse air volume once every two minutes.
Without enough ventilation, hot air can stay inside the structure and damage crops.
Operators working with greenhouse cooling in hot climate zones often use several passive strategies at the same time.
Shade Systems: Shade cloth helps block solar radiation before it enters the greenhouse. It can reduce the cooling load during hot summer days.
Thermal Mass: Water tanks or other thermal mass materials can absorb heat during the day and release it slowly at night.
Natural Ventilation: Roof vents and side openings can help hot air escape when the greenhouse structure supports passive airflow.
Evaporative cooling and air conditioning work in different ways. Evaporative cooling needs an open airflow loop. Air must enter through the cooling pads, pass through the greenhouse, and exit through the exhaust fans.
Air conditioning works best in a closed and insulated structure. It removes heat and moisture from the air, but it also requires higher energy consumption and better sealing.
Air conditioning is more suitable for small, insulated, or sealed greenhouses. It is also useful in high-humidity climates where evaporative cooling cannot provide enough temperature reduction.
The main drawback is cost. AC systems usually require higher upfront investment and higher electricity costs.
Evaporative cooling is suitable for mid-to-large greenhouses in dry or moderately humid climates. A cooling pad and exhaust fan greenhouse system can provide high-volume cooling with lower running costs than mechanical refrigeration.
If you are preparing to upgrade your greenhouse cooling system, follow these steps:
Calculate total cubic footage: Multiply length by width by average height to find the air volume of the greenhouse.
Assess summer WBD data: Check whether local humidity supports efficient evaporation during peak heat hours.
Check fan and cooling pad matching: Make sure exhaust fan CFM, cooling pad area, water flow, and controller settings match the greenhouse size.
Review infrastructure: Confirm that the electrical panel, water supply, and drainage system can support the cooling pad and exhaust fan system.
Greenhouse cooling in hot climates depends on matching the cooling method with local climate conditions. Evaporative cooling can work very well in hot and dry regions. However, it becomes less effective when relative humidity is too high.
A cooling pad and exhaust fan greenhouse system is one of the most common evaporative cooling solutions for commercial greenhouses. It pulls hot air through wet cooling pads, lowers the incoming air temperature, and exhausts warm air from the opposite side.
Start by checking local summer temperature and humidity data.
Make sure exhaust fans can provide enough air exchange.
Match cooling pad area with exhaust fan capacity.
Use shading, ventilation, and airflow control to reduce total heat load.
A: It is not recommended as the main cooling method. In high-humidity climates, water evaporation slows down, so the cooling effect becomes weak. The system may add too much moisture and increase disease risk.
A: It is a greenhouse cooling system that uses wet cooling pads and exhaust fans together. Exhaust fans pull hot outside air through the wet cooling pads. The air cools through evaporation, moves across the crop area, and exits through the fans.
A: Fogging creates very fine droplets that evaporate quickly in the air. Misting creates larger droplets that may land on leaves and floors before evaporating. Fogging is usually better for precise greenhouse cooling.
A: For hot climates, a common target is one complete greenhouse air exchange per minute. This helps remove heat and supports the cooling effect of the evaporative system.
A: Use enough exhaust fan capacity, match the cooling pad area correctly, keep the pad clean, ensure even water distribution, and reduce airflow obstructions inside the greenhouse.
