七、練習

Psychrometric properties of the air within a greenhouse give an indication of energy requirement and crop response. These properties are usually needed and have to be calculated repetitively. The following simple exercises are examples from the real world situations. EX1. Validation. Try using 3 software to run the predict-options as shown in the following table. One table for one software. Also mark N/P' for not possible to find others through this combination and mark 'N/A' for not available indicating the software does not provide this option. The column entitled '查圖' is for you to actually look into the psychromatric chart. (1 atm).
查圖1&2 1&31&41&5 .....1&10
1. dry-bulb temperature30 30.......... ...............
2. wet-bulb temperature25 25.......... ...............
3. dew point temperature..... ............... ...............
4. relative humidity..... ............... ...............
5. humidity ratio..... ............... ...............
6. specific volume..... ............... ...............
7. enthalpy.......... ............... ..........
8. saturated vapor pressure..... ............... ...............
9. latent heat of vaporization..... ............... ...............
10. vapor pressure..... ............... ...............

EX2. How to calculate the VPD (Vapor Pressure Deficit) ?

VPD = VP in leaf - VP in air
assuming: Air dry-bulb temperature = 30℃，Leaf temperature = 25, 30, 35 ℃, Relative Humidity in air is 65 %，Relative Humidity in leaf is 100%, Find: VPD = ? Pa

EX3. How to calculate the maximum reduction of dry-bulb temperature (MRdbT) in Evaporative Cooling (fan and pad) system ?

MRdbT = Eff * (WB - DB)
Assuming outside Air dry-bulb temperature is 80℉, outside relative humidity is 40, 60, 80, 100%, respectively, and the efficiency (Eff) of Evaporative cooling system is 80%.

Find: MRdbT = ?

EX4. What is dew point temperature and humidity ratio? Air dry-bulb temperature in greenhouse : 70℉，Relative Humidity is 80%，The dew point temperature is 63.57 ℉. Therefore, any surface is 63.57 ℉ or less will condense moisture from the air. Assuming the surface temperature of a PE inside the greenhouse is only 50 ℉. What is the amount of water condensed per lb of dry air?

EX5. Find water content (g/m3) of air at 18 ℃ for various relative humidities.

Note that the density of dry air at 0 ℃ and 1 atm is 1293 g/m3, but you need to find the density of dry air at 18 ℃ in g dry air/m3 dry air.
RH
HR
Water content
%g vapor./g dry air g vapor./m3 dry air
1000.01293 �}..
80�}..�}..
60�}..�}..
30�}..�}..

EX6 Study the effect of temperature on the saturated water content of air，find the difference between C1 and C2 (in g/m3 of dry air) as shown in the following table.

TemperatureDensityHR water content
0℃A1B1C1
30 ℃A2 B2C2