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 x 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).

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.

Bartok is a regular contributor to Greenhouse Management and an agricultural engineer and emeritus extension professor at the University of Connecticut. He is an author, consultant and a certified technical service provider doing greenhouse energy audits for USDA grant programs in New England.

Have a question? You can write John at jbartok@rcn.com.