Moisture is an important aspect in our incubation process, which can influence the results quite significantly. However, often we see that the discussion about moisture is based on certain assumptions that are not always valid when we look at it from a rational and physical point.
- Moisture and Relative Humidity
The terms moisture and relative humidity (RH) are often used and confused. There is a fundamental difference between moisture and RH, and this difference has a huge influence on the function of our machines. To understand how moisture influences the functioning of our machines, it is useful to point out these differences.
Moisture (water) is a liquid. It doesn’t matter how fine the droplets are that we spray, it is a liquid and not water vapor.
Water vapor is liquid in an evaporated form. To make water vapor from water we need to add a lot of energy, 2256 kJ per kg (liter) of water. To give an impression how much energy this is, it costs 5 times more energy to evaporate a liter of water than to increase the temperature of the same amount of water from 0 to 100oC. A Watt is a Joule per second, which means that if one liter of water should have to evaporate in 1 second, we would need an energy source that could deliver 2256 kW. The Mercedes Formula 1 race car engine for the year 2018 has approximately 1000 horse power, which is approximately 735 kW, so it would take that engine approximately 3 seconds to generate enough energy to evaporate 1 liter of water.
Condensation is the opposite of evaporation. This means that if condensation occurs, the same amount of energy is released. This is important, because if we have condensation on for instance cooling coils, the effectiveness of our cooling system might be less then we think, as heat is released when water vapor is condensating into water.
Relative humidity is the ratio between the amount of water in evaporated form in the air (water vapor) and the amount that the air could hold as a maximum at that temperature. At 50% RH there is enough space in the air to hold twice as much water vapor than actually is present. We often talk about moisture in the air, and then often we mean relative humidity or water vapor, but that is not necessarily always the case.
For instance, fog develops if the temperature drops so much that the capacity of the air to hold the water vapor is exceeded. As an example, air can hold almost 7 g of water in evaporated form per m3 at a temperature of 5oC. If there is more moisture in the air than 7 g, the extra cannot be evaporated and will stay in the air as little droplets, fog. If there is for instance 10 g of water per m3, the extra 3 g will be in the air as droplets. For the fog to disappear the air first have to be warmed to a level where all the water can be evaporated, which is 11oC. The fog can stay for a long time as for each m3 of air, 3 x 2,256 kJ must be provided, in this case by the sun.
Also the heat capacity of the air is related to RH, moist air can hold more heat per m3, but this has no (measurable) effect on the heat transfer, it doesn’t really change the temperature of the eggs. If there is any effect, it will be that the transfer of heat from one egg to another will go slightly easier, which will make the egg shell temperatures more homogenous, but the effects are marginal.
- Why is this important for us?
First of all: there can be more than 100% water in the air. In our example with air of 5oC we get 100% RH with 7 g of water in evaporated form, but there is an additional 3 g per m3 in the form of droplets. If our measurements show 100% RH it doesn’t necessarily mean that there is not more than 7 g of moisture per m3, but the remaining 3 g is not “measured” as RH.
Secondly, if we bring water in the air we have to provide energy before this water can evaporate. It doesn’t matter where or how we bring this water in the air, without the energy it will not evaporate. Remember the fog, without the sun it will not disappear.
If we bring water in the air, in the air handling unit, the corridor or in the machine itself, it will not be evaporated unless the energy is provided. There has to be an energy source somewhere to provide this energy. It doesnt matter if it is a fine mist or course spray, it needs 2,256 kJ of energy per g of water to evaporate. If the energy is not provided before the air enters the machine, the eggs in the machine will provide the (remaining) energy and will cool down. It is therefore an illusion to think that sprayers in corridors or in air handling units will not provide (local) cooling in the machines, unless somewhere in the installation there is a sufficient source of energy. Bringing water in from the corridor or the air handling unit will most probably change the cooling profile of the egg (and perhaps even make the distribution of the cooling more equal) compared to adding the water by a humidifier as the distribution of the water will be different.
One would expect that using steam would solve this problem, as to evaporate the water to steam the energy needs to be supplied by the steaming system. This is true, but the temperature and humidity of steam is so high that it condensates immediately when it gets in contact with the equipment (air ducts etc) during transportation of the air. This is why steam also results in droplets of water entering the machine, which still has to be evaporated.
- What does this mean for our machines?
Most people will agree that eggs and water are not a very good combination for incubation.
From one side the evaporation of water cools the eggs, and as this water is usually not evenly distributed throughout the machine, some eggs will cool more than others. This results in a poor uniformity of egg shell temperatures in the machines, a known problem.
But also from a hygienic standpoint it will not be good to have droplets of water in the machine, as with the high temperature we might stimulate bacterial growth. This is not necessarily the same as a high relative humidity. Droplets of water will stimulate bacterial growth more than high relative humidity.
This leads to the assumption that it will be better to avoid water in unevaporated form in the machines. But what does this mean for the machines itself, and then especially for the water balance and cooling.
- Water balance and cooling in the machines
When we do not evaporate water in our machines (water coming in by the humidifier, sprayer in the corridor or water in the air handling unit) we limit our total cooling capacity, as no evaporation means no energy for evaporation, so no cooling by evaporation. And 2256 kJ per liter of evaporated water is a lot of cooling, which must be replaced by more ventilation with cold air or more cooling through cooling coils.
When the cooling through the cooling coils is increased by reducing the temperature of the cooling water, more condensation is likely to occur and as said before, this limits the effect of the cooling coils itself as condensation produces heat.
If no additional water is evaporated, the RH in the machine is determined by the water vapor in the incoming air, the amount of ventilation and the evaporation of water by the eggs. When we ventilate to keep the carbon dioxide level in the machine at 3500 ppm in a single stage situation in a cold climate (approximately 0oC) and we don’t add water, the RH at day 18 will be approximately 25%. When it’s very cold weather outside, the RH will drop even a bit more, but as the amount of water doesn’t change that much anymore below 0oC, the effect will be limited.
This means that without additional water into the machine, the air in the machine will be very dry, only 25% at day 18. Most hatchery managers will not feel comfortable with these low levels of humidity. Traditionally the RH in a machine is 55%, most people with single stage machines will have not too many problems with 45%, but lower than 40% is for most people over the edge.
However, we can question what will happen if we go very low in relative humidity at the end of incubation.
- What is the effect of low RH at the end of incubation?
Although most hatchery people are instinctively afraid of going very low in RH at the end, funny enough there is not much literature or hard information available on this topic.
One of the effects is that the total moisture loss will go up, and one could expect that they embryo will dry out. However, if we look at the embryo, it is floating in the amniotic fluid, which is completely covered by the allantois, located between shell and amnion. The embryo dumps the metabolic waste water in the allantois and from there it is evaporated through the shell. That means that extra moisture loss will dry out the allantois, but not necessarily the amnion and the embryo. Even more, if we limit the ventilation in the beginning we can easily compensate for the extra moisture loss at the end. As long as we keep the total moisture loss at 18 days at the same level, it is questionable if there will be a big effect of the low humidity at the end.
One could expect that the embryo dries out at that low humidity, but as it is still in 100% water in the amnion, this is not very likely. Perhaps the allantois dries out and therefor the membranes will become too dry or too hard, but as the inside of the membrane is still 100% water, that effect will be probably limited.
With a very low humidity the loss of water from the allantois will go fast, and as minerals will not be lost with the evaporation, perhaps the osmotic pressure in the allantois will change faster than the embryo can handle. As far as I know there is no literature on that available, but if it would happen there would probably be an effect on the kidneys of the embryos, which might result in an increase in broiler mortality, perhaps not in the first week already but perhaps in the second week, as kidney failure will need time to express itself in mortality.
However, we should not underestimate the effect on the cooling of the machine. Very low relative humidity means no extra cooling by evaporation, which means that the cooling coils have to be able to do the cooling needed.
But also, a lower relative humidity means less condensation in the machine. The dew point for air of 37oC and 55% RH is 26,4oC, so unless the incoming cooling water is 27oC, some condensation will occur. But with 27oC cooling water a lot of water is needed to get enough heat out of the machine by the cooling coils. However, if we drop the relative humidity to 25%, the dew point at 37oC is only 13,7oC, which means that less or no condensation will occur on the cooling coils.
There is not a lot known about the effect of very low relative humidity at the end of incubation, but as there is a significant effect of bringing droplets of water in the machine on temperature distribution and perhaps also on bacterial contamination levels, it is worth to evaluate the effect of limiting the sprayers, wherever they are in the system.