Greenhouse Temperature Control: Gases, Ventilation, Heating, Circulation, and Cooling - Pr, Study notes of Gardening and Horticulture

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Fall 2002 Biernbaum, MSU HRT 221, pg 25 HRT 221 GREENHOUSE STRUCTURES AND MANAGEMENT PART II --GASES AND TEMPERATURE Gases in the greenhouse carbon dioxide, oxygen, water vapor Temperatures for plant growth What is "heat" and how is it measured? --VENTILATION Temperature control Humidity control Carbon dioxide levels Passive venting Forced or active venting (calculations) --EVAPORATIVE COOLING Advantages of evaporative cooling Principles of adiabatic (evaporative) cooling Calculations and system design --HEATING Calculating heat requirements Methods of heating central systems - steam, hot water localized systems - forced air, radiant heaters Selection of a fuel source Emergency heaters and generators --CIRCULATION Overhead unit heaters with fans, or fan jets Overhead units with perforated convection tubes Horizontal air flow (HAF) --TEMPERATURE CONTROL SYSTEMS Types of hardware Concepts of Staging --ENERGY CONSERVATION IN GREENHOUSES Low, medium and high cost methods Carbon dioxide fertilization Fall 2002 Biernbaum, MSU HRT 221, pg 26 Part II - Introduction (Lecture 1) Now that we know something about the structure and light levels in the greenhouse, we need to learn how to maintain the desired temperature and conditions for optimal plant growth and quality. The Greenhouse Environment LIGHT TEMPERATURE GASES (CO2, O2, H2O vapor) FERTILIZER ROOT MEDIA WATER We have talked mainly about light. The lectures for the next three weeks will cover the control of the greenhouse atmosphere. (Water, media and fertilizer will follow.) The greenhouse is subject to the extremes of summer heat and winter cold. There is little or no insulation or buffering capacity and the temperature can change dramatically, dependent on the level of sunlight. The atmosphere must also contain the proper amounts of water vapor, carbon dioxide, and oxygen. Our basic goal is to provide conditions which allow photosynthesis and respiration to proceed so that our plants grow, and that the plants look good and are marketable. Photosynthesis Respiration Fall 2002 Biernbaum, MSU HRT 221, pg 29 Ventilation (Lecture 2) Fresh air must be brought into the greenhouse for 1. Temperature control and cooling 2. To renew the carbon dioxide supply 3. To renew the oxygen supply 4. Humidity control 1. Temperature control and cooling As we discussed earlier, the greenhouse is a solar collector which warms up dramatically during the day, even on a cold day. In high light areas of the country, it may not be practical to grow plants in greenhouses during the summer without shading because temperatures are so high. Traditionally greenhouse temperatures were allowed to increase during the day to take advantage of the free heat energy. As we have learned more about the effect of temperature on plant growth, we realize that higher day temperatures influence plant height and other factors. The ability to accurately control day temperatures year round may give northern greenhouse growers an advantage that offsets the higher heating costs during the winter. For cooling or temperature control, the amount of ventilation depends on: solar intensity outside temperature greenhouse characteristics nature of the crop The goal is to keep the temperature in the greenhouse close to the setpoint and uniform over time. The amount of variation about the set point is called the dead band. Fall 2002 Biernbaum, MSU HRT 221, pg 30 2. Renew the carbon dioxide supply (Nelson, chapter 9, pg 335) Under high light and temperature conditions, the CO2 level in the greenhouse can be decreased significantly by photosynthesis which in turn can limit growth. Either fresh air must be introduced or bottled or generated CO2 can be injected into the greenhouse. During the winter, venting may be required for CO2 even though the heat is in use. As the cost of heating increases, the cost of bottled or generated gas could offset the cost of ventilating and heating and it would be more efficient to add CO2 to the greenhouse. Data (graph) of CO2 draw down during the day. 3. Humidity control - a high moisture environment is conducive to disease multiplication and spread. Relative Humidity (RH): the amount of moisture in the air relative to the total amount that can be held at a given temperature. For each 20 degree rise in temperature, the moisture holding capacity is doubled. For each fall of 20 degrees the moisture capacity of air is cut in half. One degree rise in temperature, 2.5% drop in RH; assuming the moisture content stays constant. Example Warm moist air in the greenhouse Temperature drops as the sun goes down Relative humidity increases rapidly Condensation of water occurs on the plants The solution is to remove warm moist air (vent), draw in cool air, and then heat the cool air, reducing the relative humidity. Relative humidity can also be reduce by heating first, and then venting out the warm moist air. Fall 2002 Biernbaum, MSU HRT 221, pg 31 Example: (Using a psychometric chart pg 142, figure 4-2 and a psychrometer.) 4. Renewal of the O2 supply for combustion of heater flames (not a common problem) Low oxygen is dangerous for employees and plants. With poor combustion, ethylene, carbon monoxide, or other harmful products may form. If unit heaters are not properly vented, the O2 levels may get dangerously low, particularly during very cold periods when the heaters run for long periods of time. There are seasonal differences in how ventilation is accomplished. winter - introduce small quantities of cold air with adequate mixing summer - exchanging large quantities to minimize the temperature rise due to solar input. spring/fall - alternating need for small and large quantities relative to weather conditions. Ventilation can be accomplished by either passive or active methods Natural or Passive venting Warm air in the greenhouse rises, cool air sinks. Vents are opened like windows to release the warm air. Diagram: The vent area should be at least 16 to 30% of the floor area. Example: Fall 2002 Biernbaum, MSU HRT 221, pg 34 Fan placement 1. Fans should be placed on the leeward or down wind side of the greenhouse with the vents on the prevailing wind or windward side. 2. Fans should not blow on other vents closer than 50' away. 3. Fans should not blow at each other. The can be located on opposite walls if they are alternate or off center. If fans are blowing at a wall, the distance from the fan to the wall should be at least greater than 1.5 times the diameter of the fan. 4. The distance from the fans to the vents should be between 100' and 200'. Greater than 200' requires more, large fans and is expensive. Less than 100' influences the velocity factor (F vel) and may result in areas of high air flow or low air flow. 5. Fan blades must always be screened off for safety. 6. Most fans are designed to operate at a static pressure of 0.1". If the vents are too small, the fans will create a vacuum in the greenhouse and the fans will work too hard. Diagram: Fall 2002 Biernbaum, MSU HRT 221, pg 35 Evaporative Cooling (Lecture 3) Nelson, Chapter 4 Evaporative cooling can be used to cool the greenhouse under high temperature and high light conditions. Evaporating water absorbs large amounts of heat energy. The air entering the greenhouse can be cooled the same way your body is cooled by water evaporating from your skin when you get out of a pool or lake after swimming. Advantages of evaporative cooling: 1. quality crops year round 2. plants on schedule (explain heat delay) 3. permits higher light intensity 4. may reduce watering requirement 5. moist air during the day is suitable for plants 6. more comfortable employee working conditions Background With evaporative or adiabatic cooling, the amount of energy in the air stays the same but the temperature changes. With the evaporation of water, heat energy that can be sensed by your body or a plant (sensible heat) is converted to heat energy in water vapor (latent heat). Sensible heat is measured by the dry bulb temperature. If the sensible heat is used to evaporate water, the dry bulb temperature of the air will drop until it reaches the wet bulb temperature. The wet bulb temperature is the temperature at 100% relative humidity or when the air is fully saturated. The difference between the dry bulb temperature and the wet bulb temperature is a measure of the ability of air to hold more water. This difference is also used to calculate relative humidity. With evaporative cooling, the air is saturated by passing it through cooling pads which are placed on the side of the greenhouse opposite the fans. Example: Cooling pads were once made exclusively of excelsior or aspen; fine strips of wood gathered into a pad. The most common material today is a crossfluted cellulose (cardboard like) material that Fall 2002 Biernbaum, MSU HRT 221, pg 36 is more efficient at saturating the air. The cellulose pads come in a variety of sizes and widths, as well as some with fiberglass. There are also aluminum pads available. The initial cost and replacement rate vary. The pad area needed is also different. One square foot of excelsior pads are required for each 150 CFM. One square foot of 4" crossfluted cellulose is required for each 250 CFM. See Figure 4-3 on page 149 for design. Calculations The calculations are a continuation of those used to determine the size of the fans needed for summer cooling. 3. Determine the cooling pad area by dividing the total number of CFM for one air exchange by the CFM/square foot of pad area. The amount of pad area can also be determined from Table 4-5 on pg 147. 4. The pad area divided by the length of the wall to be covered will give the height of the pad needed. 5. Water must be pumped to the pads at the rate of 0.33 gal/min/foot of pad length for excelsior and at the rate of 0.5 gal/min/foot of pad length for 4" cellulose. The number of gallons per minute calculated can be used to select a pump of the proper size. It is recommended that one pump not supply any more than 60 foot of pad. More pumps should be used for longer sections. 6. A reservoir must also be provided to supply water for the pump and to collect water when the pump shuts off. The reservoir or sump should hold 0.75 gallons of water per square foot of 4" cellulose pad or 1.5 gallons of water per foot of pad length of excelsior. The cooling pad efficiency is dependent on the density of the pad, the thickness of the pad, and the accumulation of salts from the water, dirt, or algae. Excelsior or aspen pads only last 1 to 2 years, cellulose pads should last 5 to 10 years.