Evaporative cooling systems are based on this effect: spraying water reduces temperature and therefore cools the egg mass in a machine. And if problems occur with overheating – we can further reduce temperature by increasing the amount of water sprayed and adjusting the humidity settings.

However, by looking more closely at the total system, the cooling effect of evaporation – and the energy that must be used to achieve it - we find factors that should be taken into account.

How much water is needed for effective cooling?

If a machine contains 100.000 eggs each weighing 60 grams, total egg mass is 6000 kg. The specific heat of eggs is approximately 3.5 kJ per degree Celsius per kg, so to cool the egg mass in the machine by just 1degree Celsius, we must extract (6000 x 3.5 =) 21,000 kJ or about 5000 kcal of energy from this egg mass.

The evaporation of water uses energy at a rate of 2260 kJ per litre. Assuming a 100% effective system, where all the energy used to evaporate water is coming from the eggs, we need to evaporate almost 10 litres of water to cool the eggs by 1degree Celsius. Although this doesn’t seem like much, a machine must spray frequently to use this amount of water.

In addition, we must also consider that in reality, not all the heat for evaporation will come from the eggs, but also from the air inside the machine, and from the machine itself. However, this calculation does illustrate that the machine can cool quite substantially if a lot of water is sprayed. But will all eggs in the machine be cooled evenly?

The effect of spraying on temperature distribution

Each egg in the machine will not provide the same amount of energy for evaporation, or be cooled to the same level. The sprayer is often located close to the ventilator, so that high air velocity rapidly disperses the water. This means that the water is sprayed predominantly on the eggs in the path of water being carried by high air velocity, and these eggs will therefore provide most of the energy for evaporation - and be cooled the most. Eggs outside this path will benefit much less, or not at all.

In addition, the eggs inside this path of high air velocity lose a lot of heat through the wind chill effect, so they are already cool, and will become even colder when sprayed. Eggs away from the air velocity are relatively warm, and will remain so.

Fooling the sensors

The temperature sensor in a machine is normally located in an area of high air velocity, which is the correct location to enable the sensor to pick up temperature differences rapidly. However, if the machine sprays so much water that it isn’t all evaporated before it hits the sensor, the sensor will be cooled and the machine will think the temperature has dropped, which depending on the machine settings will start the heating programme.

It’s important to realise that for evaporation to occur, the water must first pick up (heat) energy. If it doesn’t pass enough eggs before hitting the sensor, the sensor will be cooled, although eggs outside the path of air velocity will of course become even warmer as the machine starts heating. It is like putting a thermometer in a glass of cold water to record room temperature when we feel hot: it will reduce the temperature recording, but have no effect whatsoever on the actual room temperature.

Spraying water inside machines helps to bring down the temperature, but it also brings down the temperature reading on the display, and may not have a uniform effect on the temperature of the eggs.

Trying to overcome this by spraying more water will often increase the spread in egg temperature inside the machine, and may even increase the temperature in hotspots. If a machine does not have enough cooling capacity, the spraying of water will help take out the embryonic heat. However, the spraying must be well controlled, as spraying is as much about temperature control as it is about changing humidity.

January 2007