Section 4: Calculating Greenhouse Cooling Requirements for a Standard Fan-and-Pad System
To determine specifications for cooling system, the greenhouse volume must be determined, and based upon several assumptions, a flow rate per minute (air exchanges) requirement is determined.
Determining greenhouse volume and airflow requirement
There are several ways of calculating required air flow capacities. This is one method to serve as an example. In this example, cooling specifications are outlined below for a single-span quonset greenhouse that is 30 ft wide and 110 ft in length. Some basic geometric equations allow the determination of the volume of the greenhouse.
The volume of the quonset structure is determined (assuming it is half of a cylinder) as: 0.5(πr2L)
= 0.5[(3.14)(152)(110)]
= 38,857 ft^{3}
An air exchange of 1 to 1.5 times per minute is required. The value of 1.5 would be used if the greenhouse were to be used during summer months where very high solar loads and temperatures will be experienced. In this example, 1 exchange per minute is used so that 38,857ft^{3}/min (cfm) is required.
Determining fan numbers, sizes and specifications
Fans should be spaced not greater than 25 ft apart (center to center as a general rule) and should never be greater than 200 ft from the pad wall. Therefore, this structure requires two fans spaced along the 30 ft wall. The structure will therefore require two fans with a capacity of (38,857cfm/2) 19,429 cfm each. From a list of fan specifications (taken from the table below but available from supply catalogs or manufacturer specifications), two 48-inch, 1 horsepower fans are selected. These fans provide a total of 39,200 cfm which meets the required air flow.
Determining pad wall specifications and dimensions
Aspen pads, 4-inch or 6-inch cellulose pads could be included to create a fan-and-pad cooling system. In this example, 4-inch cellulose pads are included. One square foot of this type of pad will accommodate 250 cfm. Therefore, 39,200 cfm/250 = 157 ft^{2} of pad wall is required. The pad wall should extend the entire length of the way. Therefore, the pad should be 30 ft wide and (157 ft^{2}/30ft) 5.2 ft tall. In reality, a 30ft x 6ft pad wall would be constructed (pads come in heights of 1ft increments).
Determining pump and sump tank specifications and capacities
The pump capacity must take into account the volume of water flow required by the system (pipes and pads) as well as water loss through evaporation from the pads.
To accommodate the system, 0.50 gallons/min are required per linear foot per minute. Therefore, 30 ft x 0.5 gallons/ft = 15 gallons/min to accommodate the system.
To compensate for evaporation as water moves through the pad, 0.05 gallons/min are required per 1000 cfm of airflow. Therefore, (39,200 cfm/1000 gallons/cfm)0.05 = 2 gallons/min required to compensate for evaporation.
The total pump capacity is 15 gallons + 2 gallons = 17 gallons/min.
The sump (water storage tank) capacity is determined as 157 ft^{2} of pad x 0.75 gallons/ft^{2} of pad = 118 gallons.
Air Delivery Capacities for Various Greenhouse Fan Dimensions and Motors |
||
Fan Size (inches) |
Motor (horsepower) |
Feet^{3}/min (cfm) at 0.1 inch static pressure |
24 |
1/4 |
4,500 |
24 |
1/3 |
5,700 |
24 |
1/2 |
6,500 |
24 |
3/4 |
7,600 |
30 |
1/3 |
7,400 |
30 |
1/2 |
8,800 |
30 |
3/4 |
10,200 |
36 |
1/3 |
8,800 |
36 |
1/2 |
10,600 |
36 |
3/4 |
12,700 |
36 |
1 |
14,200 |
42 |
1/2 |
12,500 |
42 |
3/4 |
15,000 |
42 |
1 |
16,800 |
48 |
1/2 |
14,700 |
48 |
3/4 |
17,800 |
48 |
1 |
19,600 |
54 |
1 |
22,900 |
54 |
1 1/2 |
25,800 |
Specifications for Design of Pad Wall for Fan-and -Pad Greenhouse Cooling System |
|||||
Pad type |
Airflow required (exchanges/min) |
Ft^{3} flow accommodated per ft^{2} of pad |
Water flow rate (gallons per minute per linear ft) |
Water loss rate (gallons per minute per 1000 cfm of air flow) |
Sump capacity (gallons/ft^{2} of pad) |
Aspen |
1 to 1.5 |
150 |
0.30 |
0.05 |
0.50 |
4" cellulose |
1 to 1.5 |
250 |
0.50 |
0.05 |
0.75 |
6" cellulose |
1 to 1.5 |
350 |
0.75 |
0.05 |
1.00 |