Basic greenhouse engineering calculations

Several basic equations are used to make calculations when designing environmental control systems for greenhouses. These are not very complicated and will give a grower information that can be useful when setting up or remodeling a structure.

Heat loss

The heat loss for a greenhouse is estimated by multiplying the surface area, a heat transfer coefficient and the air temperature difference between inside and outside. This equation assumes no solar gain and is used to calculate the size of the heating system needed for the coldest night.

HL = A x U ΔT Where: HL = Heat loss (Btu/hr). A = Surface area of greenhouse (sq. ft.) – includes roof, sides & endwalls unless these are different materials. U = U-value — Heat transfer coefficient (Btu/hr – sq ft-°F) — Use 1.25 for single layer poly, polycarbonate or glass, 0.8 for double-layer poly, 0.7 for double-wall polycarbonate and 0.6 for triple-wall polycarbonate. These values take into account an average infiltration heat loss. ΔT = Temperature difference between inside and outside (°F).

* rafter length from eave to eave (add two times sidewall height) ** hoop length from ground to ground *** hoop length from gutter to gutter (add sidewall height on exterior walls) **** Approximate area (high tunnels have larger endwalls) - multiply by two for both endwalls Example: 30’ x 100’ gothic style hoophouse - 60°F inside night T, -10°F outside night design T. Greenhouse surface area = 45’ x 100’ + 2(255 sq ft) = 5010 sq ft HL = 5010 sq ft x 0.8 x 60 – (-10) = 280,560 Btu/hr

Surface area (A)

Greenhouse surface area = the distance over the top of the greenhouse from ground to ground x length of greenhouse + endwall area. The values in this table may vary slightly depending on the manufacturer or sidewall height.

Roof/sidewall vents or louvers

American Society of Agricultural and Biological Engineers standards recommend that the combined roof vent area should equal the combined sidewall vent area, and each should be at least 15 to 20 percent of the floor area. For northern climates, 15 percent may be enough but greater amounts are needed in warmer climates. In large gutter-connected greenhouses, where sidewall vent area per floor area decreases as the width increases, larger roof vents are needed. Open roof designs may eliminate the need for sidewall/endwall vents where more than 50 percent of the roof is open.

Fan ventilation

Total fan capacity* (cubic feet/minute) = 8 x the greenhouse floor area (square feet)

*fan capacity is measured at 0.10 - 0.125 inches water static pressure Note: use a large diameter fan with the smallest motor for the highest efficiency.

Example: 30 foot x 100 foot hoophouse Total fan capacity needed = 8 x 30’ x 100’ = 24,000 cfm

This could be supplied by two 42 inch fans with .25 horsepower motors or two, 36-inch fans with .5 horsepower motors.

Horizontal air flow (HAF)

Total HAF circulating fan capacity (cubic feet/min) = 2 x greenhouse floor area (square feet).

Fan capacity is measured at 0 inches water static pressure

Example: 30’ x 100’ hoophouse HAF = 2 x 30’ x 100’ = 6,000 cfm Install four – 12” diameter, 1/10 horsepower circulating fans with a capacity of 1550 cfm each.

USDA has developed the Virtual Grower software for making the calculations. It has formulas for additional calculations.

It is available at goo.gl/nvO1cW.

December 2015
Explore the December 2015 Issue

Check out more from this issue and find your next story to read.