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ENERGY


The Mollier diagram and air handling


The article in last issue used three examples to clarify the Mollier diagram (diagram of moist air). This article completes the subject by using the diagram to explain the effects in the climate unit during


cooling, heating, humidifying and dehumidifying. By Jan Gielen, DLV Plant Mushrooms Manager/Climate & Energy Specialist j.gielen@dlvplant.nl


T


o illustrate the examples, this article is based on the assumption that optimal management of a certain cultivation situation could require an inlet air climate of 17° C and 75 % RH– in other words a moisture content of 9 g/kg and a heat content of 40 kJ/kg. How the climate unit achieves this desired climate condition (preferably energy-efficiently), depends on the computer control program and the CO2 set by the grower.


limits Summer conditions


Example 1 of the Mollier diagram is based on two summer conditions: warm and moist (climate point 1) and warm and dry (climate point 5).


Cooling, dehumidifying and re-heating As, especially in the Netherlands, the ‘warm and humid’ condition occurs more frequently, this example will be explained first. As a starting point, we will take a climate condition that can occur in the air mixing box of a climate unit in such a situation. To proceed from mixed air condition 1 (25°C, 60% RH, 12 g/kg moisture content and 56 kJ/kg heat content) to inlet condition 4 (17°C, 75% RH, 9 g/kg moisture content and 40 kJ/kg heat content), we start by cooling (line 1-2). At point 2, the air behind the cooling coil has been cooled to 16.8°C. This point is referred to as the dew point as the air has now reached 100% RH, but still has a moisture content of 12 g/kg. The moisture will only condense on the cooling coil surface if we continue cooling to below this dew point (line 2-3). To reach the desired moisture content of 9 g/kg, we must cool the air behind the cooling coil down to 12.5°C (climate point 3). We now


20 MUSHROOM BUSINESS


have the correct moisture content, but the air is too cold and must be re-heated again to reach the desired inlet condition (line 3-4). This re-heating reduces the RH, but the moisture content remains unchanged. In this way, the desired inlet condition 4 (17°C, 75% RH, 9 g/kg moisture content and 40 kJ/kg heat content) can be achieved.


Adiabatic cooling (desert cooling) We are unlikely to experience this in the Netherlands, but in a continental climate warm and dry climate conditions can occur (climate point 5). Adiabatic cooling is necessary to directly reach inlet condition 4 (17°C, 75% RH, 9 g/kg moisture content and 40 kJ/kg heat con- tent), coming from mixed air condition 5 (25°C, 28% RH, 5,7 g/kg moisture content and 40 kJ/kg heat content). Adiabatic cooling is a system that draws air through wet desorption pads that are kept moistened by water. This method simul- taneously cools and humidifies the air. To reach


Example 1: Mollier diagram with two summer conditions.


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