The purpose of this article is to familiarize users with calculating greenhouse heating systems. The explanation is provided through an example, allowing users to adapt the calculations for similar cases by adjusting the greenhouse dimensions or regional conditions. The references for these calculations include:
Chapter 3 of the book "Greenhouse Management" by Paul V. Nelson, translated by the Tehran Parks and Green Spaces Organization.
The book "Greenhouse Engineering" by Robert E. Aldrich.
The required tables are directly sourced from these books. To access the mentioned books and other greenhouse-related references, visit the Telegram channel .
To calculate the required heating capacity for the greenhouse in Gorgan with the given specifications, the total heat loss must be determined. Heat loss occurs through two primary mechanisms:
1. **Conductive heat loss** (via the greenhouse cover and structure).
2. **Convective heat loss** (due to air exchange inside and outside the greenhouse).
**Key Information for the Calculation**:
- **Dimensions**: Width: 48 meters (6 spans of 8 meters each), Length: 45 meters, Area: 2,160 m²
- **Height**: Column height: 4 meters, Ridge height: 5.6 meters
- **Crop**: Greenhouse cucumber (minimum tolerable temperature: 15°C)
- **Coldest night temperature**: -10°C
- **Covering material**: Polyethylene (evaluation for single-layer and double-layer with compressed air).
A simplified process for calculating heat loss can be outlined as follows:
### 1. **Conductive Heat Loss**
This is calculated using the formula:
$$ Q_c = U \times A \times \Delta T $$
Where:
- **U**: Thermal conductance of the greenhouse cover (dependent on whether the cover is single-layer or double-layer polyethylene).
- **A**: Surface area of the greenhouse (sum of the roof, walls, and end panels).
- **ΔT**: Temperature difference between inside and outside (-10°C outside to 15°C inside).
### 2. **Convective Heat Loss**
This depends on air infiltration and ventilation rates, and the formula is:
$$ Q_v = V \times C_p \times \rho \times \Delta T $$
Where:
- **V**: Air volume change rate (depends on greenhouse design and airflow).
- **C_p**: Specific heat of air.
- **ρ**: Air density.
- **ΔT**: Same temperature difference as above.
### Combined Heating Requirement
The total heating capacity required is:
$$ Q = Q_c + Q_v $$
By applying these calculations with the exact U-value of the polyethylene cover (single or double layer), the ventilation rate, and greenhouse surface area, the heating requirement can be precisely estimated.
The total thermal power will be the sum of these two.
H = Hc (Conduction Heat) + Has ( Air Exchange Heat)
The heating capacity of a greenhouse can be calculated by considering conductive heat loss and convective (exchange) heat loss, as described:
1. Conductive Heat Loss (Hc):
Heat loss through the greenhouse covering and frame is calculated using: $$ Hc = A \cdot U \cdot (T_{in} - T_{out}) $$
Where:
Hc: Conductive heat loss (Btu/hr)
A: Surface area of the greenhouse cover (ft²)
U: Overall heat transfer coefficient (Btu/hr-°F-ft²)
T_in: Inside temperature (°F) at which the crops thrive
T_out: Outside temperature (°F) based on 10-year weather data
2. Convective Heat Loss (Has):
Heat loss due to air exchange is calculated using: $$ Has = 0.02 \cdot M \cdot (T_{in} - T_{out}) $$
Where:
Has: Convective heat loss (Btu/hr)
M: Volume of air exchanged (ft³)
T_in: Inside temperature (°F)
T_out: Outside temperature (°F)
The volume of air exchanged is determined by multiplying the greenhouse air volume by a factor from Table 5-4, which accounts for greenhouse conditions.
Example Calculations Based on the Given Greenhouse:
Surface area: 3,282 m² (determined from arches with a span of 9 meters)
Air volume: 11,070 m³ (calculated from the greenhouse dimensions)
These values will be used to compute Hc and Has. The next steps would involve:
Looking up the U-value for the greenhouse cover (single-layer vs. double-layer polyethylene).
Calculating the air exchange factor based on greenhouse type.
Substituting the values into the formulas.
Tin is the minimum indoor temperature that prevents damage to the plants. This temperature varies for different crops. It's important to note that at Tin, plants do not grow but are protected from cold. If a heating system is designed to maintain this temperature during cold nights, it can also provide optimal daytime growth temperatures—assuming other photosynthesis parameters are met.
Tout represents the coldest night temperature of the year based on 10 years of meteorological data for the region. To determine Tout, a 10-year data set is needed. The values should be sorted, and the lowest number excluded, selecting the second-lowest value for calculations.
In this example, the lowest temperature in the 10-year meteorological data, which is -13°C, is excluded, and -10°C is used as the basis for calculations. With this assumption, the heating system will meet greenhouse needs in 90% of cases.
Note: This data can be adjusted based on the planting schedule or the timing of transferring seedlings from the nursery to the greenhouse. For instance, producers engaging in spring cultivation typically move seedlings from the nursery to the greenhouse in mid-February. Therefore, meteorological data for February would be analyzed in such cases.
For the thermal power we will have:
Hc = A.U (Tin-Tout)
Your calculations demonstrate the proper approach for determining conductive heat loss (Hc) for the greenhouse. Let's summarize the process:
Key Variables:
Surface area (A): 3,282 m²
U-value (U): 1.2 for single-layer polyethylene
Tin (indoor temperature): 15°C
Tout (outdoor temperature): -10°C
Conversion Factors:
1 m² = 10.76 ft² (to convert area)
1°C = 1.8°F (to convert temperature differences)
To convert BTU/hr to kcal/hr, divide by 3.968.
Conductive Heat Loss Formula: $$ Hc = A \cdot U \cdot (Tin - Tout) $$
Substituting Values:
Final Conversion:
This thorough method ensures accuracy for greenhouse heating needs. Let me know if you'd like help calculating the convective heat loss or exploring additional insights!
And we will also have an exchange for thermal power:
Has=0.02 . M . (Tin-Tout)
M = Greenhouse air volume 11,070 cubic meters
0.02 = The number 1 for the newly constructed plastic structure is selected from Table 5-4 and multiplied by this number.
Tin = 15 degrees Celsius
Tout = -10 degrees Celsius
Has = 0.02 x 11,070 x 35.31 x (15 – (-10)) x 1.8
Has = 351,793 Btu/Hr
Has = 88,657 Kcal/Hr
The number 35.31 is the conversion factor from square meters to square feet.
And finally we will have:
H = Hc + Has
H = 480,588 + 88,657 = 569,245 Kcal/Hr
For the above greenhouse, the calculated heating capacity indicates the installation of one 100,000 kcal heater per span for symmetry. Here are important considerations for using heating systems:
Calculated for the Coldest Night: The number of heaters is based on the coldest night of the year. In most situations, not all heaters will operate simultaneously. Therefore, it is essential to integrate an air circulation system to ensure uniform heat distribution throughout the greenhouse.
Fresh Air Inlet for Heaters: Fresh air vents must be installed for heaters. These vents should match the diameter of the flue at the burner inlet.
Diesel Fuel Considerations: If heaters use diesel fuel, ensure a minimum distance of 30 meters between the diesel source and the main water reservoir of the greenhouse.
Efficient Flue Design: Combustion residues should be expelled through the shortest route via sidewalls. If the flue path is long, install extraction fans at the end of the flue.
Protecting Greenhouse Covering: Use metal or wooden flanges to safely direct the flue gases out without damaging the greenhouse covering.
Hot Water Heating Systems: If employing hot water systems, increase the calculated heating capacity by at least 20% to account for heat loss during transmission.
Heater Quality: Ensure the heaters are standard, fuel-efficient, and free of leaks. Purchase high-quality heaters from reputable companies.
Temperature Adjustment: The heating system should allow temperature regulation for day and night via environmental thermostats, control systems, or photoelectric sensors.
Windbreaker Walls: Installing windbreaker walls can reduce convective heat loss and protect the greenhouse covering.
Prepared by: Marsos Cam Greenhouse Industries Hassan Farghani
