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Energy consumption air resistance E = (q x ∆p x h) / (ŋ x 1000) kWh Calculation example Airfl ow


E = energy consumption in kWh q = air volume (m3 ∆p = resistance (Pa)


/s) Resistance


Running hours Fan effi ciency


15 100


8760 0.7


h = running hours fan (hours) ŋ = fan effi ciency (usually approx. 0.7) Energy consumption 18,771


being supplied. In practice, many growers use ducts with too few, or too small, openings. This configuration achieves a high speed of air blown into the room, even though the total volume of air may still be insufficient. In rooms like this, you may notice dehydration along the edges of the lower beds (outer aisle), while bacterial blotch may appear at other places. This picture of contradictory evaporation problems indicates that air is exiting the ducts too fast, but with a total fan capacity that is too low.


The best results are generally achieved if suffi- cient volumes of air are blown into the growing room at a relatively low speed. This is particularly important on manual harvesting farms in view of good quality. Blowing a large volume of air into the growing room at low speeds is only possible if the ducts are correctly dimensioned, and have enough openings. As a guideline, the surface area of all the openings should not exceed a figure equal to half the surface of the air duct.


Hidden energy consumption of fans Customary overpressure systems use a centrifu- gal fan to deliver the necessary air volume. The air resistance of the various components in the climate unit has to be considered and is partly dependant on the required volume of air. The figure applied at a manual harvesting farm is maximum 22.5 m3 a maximum of 25 m3


/h per m2 /h per m2


growing surface with on mechanical


harvesting farms. There is always a certain level of air resistance in every climate unit (e.g. filters, cooling and heating coils, air distribution systems and overpressure openings). When dimensioning a centrifugal fan, a maximum air resistance of around 650 Pascal is generally taken into account. As an indication, this maximum air resistance can be broken down as follows: 350 Pascal for a soiled inlet filter, 100 Pascal for a wet cooling coil, 50 Pascal for a heating coil, 100 Pascal for the ducting and 50 Pascal for the overpressure openings.


Air resistance can rise considerably if air filters are poorly maintained, or the cooling coils are soiled. Unnecessary air resistance can also be caused by under-dimensioned installations (filters, coils, overpressure openings). Unwarranted air resistance in the climate


m3 Pa


/s hours/year


kWh


installation results in unnecessary energy consumption by the fan – which is then effectively a stealthy power waster.


Calculation example


As an example, a calculation for a manual harvesting farm with 12 growing rooms of 400 m2 surface area. The maximum fan capacity per room is: 400 * 22.5 = 9000 m3


/h. Assuming this fan runs on average at 50 % capacity, i.e. 4500 m3 /h,


the total used on the entire farm is an average of 12 *4500 = 54,000 m3 uses m3


3600 = 15 m3


/s, so the figure will be: 54,000 m3 /s.


/h. This calculation method /h /


Air duct design dictates air speed.


In this example, imagine that an average unne- cessary air resistance in the climate units of 100 Pascal occurs due to one of the causes stated above. The formula below calculates the unneces- sary energy consumption of all 12 fans. In this example, we have assumed that the fan runs all year round (8760 hours) with an average fan efficiency of 70%.


The calculation reveals that a manual harvesting farm with 12 growing rooms of 400 m2 surface area and unnecessary air resistance in the climate units of 100 Pascal, wastes roughly 18,771 kWh annually.


The calculation above can be applied as a general guideline for all situations that use a centrifugal fan. In many cases, checking for unnecessary air resistance – and making (minor) modifications – in the system can recoup much of this wasted energy.


Summary


A certain fan speed or air speed exiting the duct openings does not automatically imply that the correct volume of air is entering the room. The best results are generally achieved if high volumes of air are blown into the room at low air speeds.


This is only achievable if the fan and air distribu- tion are correctly dimensioned. Also be alert to soiled filters or coils that could cause unnecessary resistance over the system. Fans run practically 24/7, in several rooms, so unnecessary energy consumption over the course of a year can be considerable!


MUSHROOM BUSINESS 43


Soiled inlet fi lters consume energy unnecessarily.





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