the required inlet condition 4 in our example, all that has to be done is evaporate water (3.3 g/ kg). The energy needed to evaporate the water droplets is taken from the air, a process that causes the air temperature to cool (line 5-4). The water vapour and evaporative heat are released into the air again, causing the moisture content to rise while the heat content stays the same. In this way, the desired inlet climate 4 can be achieved in a single step.
Winter condition
Example 2 of the Mollier diagram is based on a winter condition and two humidification methods: steam humidification or water humidification.
Heating and steam humidification Steam humidification is traditionally the most widely-used method of RH control in growing rooms. Steam humidification is considered the most suitable method in winter, in particular when heating and humidification are required. As a starting point, we again will take a climate that can occur in the air mixing box of a climate unit in such a situation. To proceed from mixed air condition 1 (10°C, 92% RH, 7 g/kg moisture content and 28 kJ/kg heat content) to inlet con- dition 3 (17°C, 75% RH, 9 g/kg moisture content and 40 kJ/kg heat content), we start by heating (line 1-2). At point 2, the air behind the heating coil has been heated to 16.5°C. The RH has dropped to 60%, but the moisture content is still 7 g/kg. To reach the desired moisture content of 9 g/kg, steam humidification is used in this case, whereby 2 g/kg humidification still has to be added. As low pressure (0.5 bar overpres- sure) steam humidification has a heat surplus of - rounded off - 0.25°C/g/kg, 2 g/kg of low pressure steam humidification will also result in a temperature increase of 0.5°C (line 2-3), which therefore achieves the desired inlet condition 3 (17°C, 75% RH, 9 g/kg moisture content and 40 kJ/kg heat content).
Heating and water humidification If water humidification is chosen, be aware that additional heat is required to evaporate all the small droplets of water. The air must therefore be pre-heated properly beforehand.
Example 2: Mollier diagram with winter condition and two types of humidification.
In this case, 16.5°C (point 2) is insufficient, so heating must continue (line 2-4) until the air temperature reaches 22°C (point 4). The RH has dropped to 43%, but the moisture content is still 7 g/kg. To reach the desired moisture content of 9 g/kg, water humidification is used in this case, whereby 2 g/kg humidification still has to be added. As water humidification cools the temperature by – rounded off - 2.5°C/g/kg, 2 g/kg water humidification will also result in a temperature decrease of 5°C (line 4-3), which therefore achieves the desired inlet condition 3 (17°C, 75% RH, 9 g/kg moisture content and 40 kJ/kg heat content).
Note: it should be apparent that effective water humidification not only depends on small water droplets that are easier to evaporate, but also on well pre-heated air. If the air is not pre-heated sufficiently (or nor heated at all – e.g. during cool down/pinheading), the water droplets will evaporate to an insufficient degree (or not at all), which means the inlet air stays too dry and causes low RH levels inside the growing room. Many growers who use water humidification experience problems with low RH levels and scaling, certainly in winter periods. When these problems occur, growers do not often associate them with better pre-heating of the air used for water humidification, however doing this will clearly have a positive effect.
MUSHROOM BUSINESS 21
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