- Energy Saving
How to avoid condensation
Avoiding condensation on plants is a key part of any disease control strategy. Poinsettia growers have to pay particular attention to this problem as they use a technique to control plant height called ‘Drop’. ‘Drop’ involves reducing the greenhouse temperature to as low as 12 degrees C before sunrise and is followed by an increase in the greenhouse temperature to the daytime set point within one hour after sunrise.
The risk of condensation is greatest during the period when the greenhouse temperature is increased; if the plants do not warm up as quickly as the surrounding air, condensation can occur.
In practice, this is a common problem as cold air can be trapped within the crop canopy causing the plant temperature to stay lower than the air above it. The warm air above the crop comes into contact with the cold plant and moisture is released.
Similar problems can occur with all crops, so the techniques described below are universally applicable no matter what you grow.
Using plant temperature
Condensation occurs when the air temperature falls below its ‘dew-point’; so to stop condensation, we need to make sure that the plant temperature is never lower than the dew-point of the air.
Ideally, we would like to measure the temperature of the leaves, stem etc. and then use this to control greenhouse environment. An infra-red camera can do this, but these devices are expensive and they need to be used with care. For example, infra-red cameras were used to measure the plant temperature of poinsettias as part of HDC Project PC 207 and it was found that the measured temperature was affected by the stage of plant growth. When the crop was young, the infra-red camera measured the ‘average’ temperature of the leaves, stems, pots, greenhouse floor, etc. When the plants were fully developed, only the leaf temperature was measured. It was therefore concluded that, whilst infra-red cameras offered a sophisticated solution to measuring plant temperature, their use, in practice, was not straightforward.
An alternative to taking direct measurements from the plant is to use a computer model to predict the temperature. A number of the latest greenhouse control computers provide this facility and it can be used to highlight when condensation risks are highest.
The greenhouse climate last week
This graph shows a day at Millbeck Nursery following implementation of our first set of climate control settings. At first glance, the greenhouse climate looks acceptable — however, if we add some additional information, potential problems are revealed.
Click on graph to see a larger version
The graph below shows the same period, but in addition to the greenhouse temperature (in green) two other lines have been added:
1. The calculated plant temperature (in pink) and;
2. The dew point temperature of the greenhouse air (in brown).
These lines show that the calculated plant temperature was the same as the dew point temperature between 10:00 and 10:30; so at this time the risk of condensation on the plants was extremely high.
Whilst we can’t say conclusively that condensation occurred at this time, this graph tells us that the risks were high.
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Why was the condensation risk high?
- High RH; so the moisture holding capacity of the greenhouse air was very close to saturation.
- The plants were yet to warm up following the low temperature ‘Drop’ treatment.
So what are the solutions?
- Give the plant more time to warm-up before allowing the differential between the heating and ventilation temperatures to increase.
- Lower the RH during the warm-up period by using more heat.
To implement option 1, introduce higher ventilation temperatures gradually. This effectively delays the start of the daytime heating and ventilation set-points (i.e. Period 3 — see Good humidity control with minimum energy use). This strategy avoids the use of any additional heat.
With option 2, an increase in pipe heat during the transition period is needed, so energy costs will be higher.