Heat loss: the Reason for Heating

Transcription

1 Greenhouse Heating

2 Heat loss: the Reason for Heating Greenhouses loose heat quickly Materials used that need to transmit light don t have very good insulative qualities Northern greenhouses can loose more than 100 BTUH or more per square foot at low temperatures

3 Heat Transfer

4 Greenhouse Heating Terms BTU British thermal unit = the amount of energy it takes to heat 1 pound of water 1 degree F BTUH The quantity of BTU s (British thermal Units) transferred in one hour (Most heaters and boilers carry a BTUH capacity) Delta T ( T) is the Difference between 2 temperatures. Usually between inside and outside temperatures U Value - The inverse of the commonly understood R value when referring to insulation. U value is used to compare ability of a material to transfer heat.

5 Calculating Heating Needs Lets say you have a greenhouse sidewall that is 12 Tall X 100 Long Square footage = 1,200 sq. ft. It is made of double inflated polyethylene that has a U value of 0.7 You have an outside temp of 0 degrees F and are trying to maintain an inside temp of 70degrees F What is your T? 70 *Formula 1,200(square footage) X 0.7(U value) X 70 ( T) =? 58,000BTUH

6 Types of Heat loss Convection (through the greenhouse covering) the most common and least controllable form of heat loss Infiltration influx of cool air through the cracks or spaces in panels or walls into the greenhouse. This becomes much more critical in areas where temperatures drop to much lower levels than we see in Southern California. It is however where we can improve conditions by sealing openings, cracks or holes that exist

7 Types of Heating Systems Unit Heater Systems Heat distribution convection tubes Heat Distribution HAF fans Central Heating systems (Boiler) Wall, Over head, and Bench pipe configurations Box coil vs. Trombone coil Floor pipe coil Radiant heat Solar heating Systems

8 The Two Parts of a Heating System Heat Source The burner, fire box or radiant heating element usually running on some sort of fuel provided. Distribution System The method of transferring the heat produced by the heat source as equally as possible throughout the inside of the greenhouse. Most common methods are using water, steam or heated air moved by fans.

9 The Unit Heater The unit heater is probably the most common form of heating in our area. They consist of a burner in a combustion chamber, heat exchanging tubes, a smoke stack for venting fumes away and a fan to push air pass the heat exchange tubes and out into the greenhouse They can be thermostat driven within the unit or through sensors within the greenhouse directing the heater to come on at temperature presets. Typically run on natural gas or fuel oil

10 HAF Horizontal Air Flow Fans Horizontal air flow fans are responsible for equalizing temperatures throughout the greenhouse by creating a circulation of air. They are usually in place every 50 feet in a large greenhouse, parallel to the ground and mounted 2 to 3 feet above the plants. They must be built for heavy duty use since they are responsible for working in conjunction with both the heating and cooling system and are key for crop success

11 HAF Fan Configuration The circulation pattern of the half fans is adjusted to move air between 50 and 100 feet per minute. Less than 50 fpm and air movement is erratic and uniform mixing of air cannot be guaranteed. Higher than 100 fpm and too much energy is being expended. The first fan is no closer than 10 to 15 from the end wall and are run in opposite directions from the fans on the other side of the single greenhouse to create air circulation In larger gutter connected greenhouses you are creating multidirectional circulation patterns

12 Heat Distribution Convection Tubes Heat convection tubes work in conjunction with a inflation fan that inflates the polyethylene tube extending it the length of the greenhouse. The tube has holes arranged to distribute the warm air towards the crop. The holes are spaced at intervals and intended to release warm air out evenly across the growing area. The inflation fans have an intake that accepts the warm air coming from the heater then redistributes that air into the inflation tube

13 Central Heating (Boiler) A central boiler system is used in a large greenhouse or greenhouse range of connected greenhouses. It is also the preferred system in more temperate climates. The central boiler is selected to run on the fuel that is the most economical for the area (typically natural gas or fuel oil). Primarily heating water which is then pumped throughout the growing space either in the side walls, overhead, under the benches, radiating from within the floor, or a combination configuration. A small pump moves the heated water through the piped system and back through the boiler for reheating.

14 Why Boiler Systems are the Most Common System Globally Water can carry 3,500 times the energy that air can carry 1 cu.ft. Water 3,200 cu.ft. Air

15 The Boiler Considered the heart of all hot water systems, boilers have many different designs mostly related to how the heat from the burner is transferred to the tubes inside the boiler. EX: Fire-tube boilers, Water-tube boilers and Cast-iron section boilers. The Demand-type boiler systems are the latest and considered the most energy efficient systems. Somewhat resembling a tankless hot water heater in the home the water flows past the heater element that only comes on when a demand from the greenhouse is needed. Allows for expansion and tiered heating.

16 Fuel Systems Heating oil Natural Gas Propane Biomass systems This decision is made based on proximity to fuel sources, greenhouse heating component choices, and cost

17 Biomass Systems A Biomass system is one that relies on a stream of off cast material that can be burned for fuel and heating Allows you to get off the grid Takes advantage of waste products Wood pellets as fuel

18 Considerations with Biomass Systems Is your fuel really free or cheap? Generally 3 times the maintenance as gas or oil systems Fuel needs to be handled, and inspected for quality variances Fuel needs to be stored Emission laws are tightening up Heat storage tank with vertical stratification for varying temps Distribution system needs to be efficient There are incentives in local, state, and municipal areas to help offset costs

19 Wall, Overhead, or Bench Pipe Configurations Heating pipes distributing radiant heat are placed in 1 to 4 areas. 1. Along the end walls of the greenhouse reducing the negative impact from pipes blocking the solar radiation. 2. Overhead in the area that cools off the fastest from convection compensating for heat loss. 3. Under the benches where the heat can then rise to the crop above where it is out of the way. 4. From the floors themselves creating a uniform heat but costing the grower a significant front end cost. Warm water pipes distribute heat below the benches where it rises to the crop above Typically the grower uses a combination of these configurations depending on his geographic location and budget.

20 Piping Configurations for Boiler Systems The arrangement or configuration of pipes carrying warm water is calculated based on the climatic demands of the site, along with the needs of the crops being grown. Having multiple circuits or even heating sources that can be regulated offers some flexibility with fine tuning your heat distribution within your system

21 Piping Configurations for Boiler Systems A box configuration or manifold system allows for more even temperature throughout the system In a trombone system the heat at the source is significantly warmer at the beginning than at the end of that long length of pipe Box configuration Trombone configuration

22 Floor Heating System Radiant floor heating systems are built early in construction and allow for even heat distribution within the greenhouse. Initial cost is high and concrete floors must be insulated from the ground and pipes placed at precise levels within the concrete pour to work properly. Pipes instead of metal are often a PEX tubing which is a flexible and durable plastic pipe. Heat distribution is even as the concrete slab becomes the source of radiant heat Small concrete pad with Pex tubing to distribute heat

23 Infrared Radiant Heating Systems These heaters emit infrared radiation which travels to an object that it is pointed and heats that object such as the plants or the sidewalk. The air through which the radiation travels is not heated so the greenhouses heated with this method can remain cooler overall. Less differential between inside and outside temps means less heat loss in the system Fuel costs are reportedly less with these systems due to more heat being derived from the combusted fuel which is claimed to be about 90% efficient

24 Electrical Resistance Systems Usually for spot controlled areas within greenhouses or special propagation greenhouses. Consisting of a rack-like grid or mat at the bottom of the pots to increase the soil temperature within through convection Usually only efficient in areas under 100 sq. ft.

25 Solar Heating Still in early stages of efficiency the solar heated greenhouse will continue to be developed They consist of 5 components: 1. Solar collector 2. Heat storage facility. 3. An exchanger to distribute solar derived heat into the greenhouse air 4. Backup heater to take over when solar heating does not suffice 5. Controls for the system.

26 Pros and Cons - Unit Heaters Pros Low costs for lots of BTU s Quick response, ease of zoning the houses. Cons Lack of focused heating (needs distribution system) longevity can be poor in greenhouse environment Can cast shadows on planted areas Can be noisy Limits ability to hang plants near heaters

27 Pros and Cons Hot Water Systems Pros The Most Flexibility Manageability Ability to change heating fuel without changing distribution pipes Cons Initial Costs are High Complexity Special skills needed for initial installation Shadowing of plant material

28 Pros and Cons Infrared Systems Pros Energy efficiency Easy installation Leaves are kept dryer Reduction in diseases such as botrytis Cons Shadowing Proximity dependent heat transfer Uneven heat transfer

29 Rough Costs of these Systems Unit Heater $0.40 to 0.80 per square foot Hot Water System $1.50 to 4.00 per square foot Infrared System $1.00 to 1.50 per square foot

30 The Take Away Message There are a huge amount of choices for heating greenhouses. The choices are made considering location, common climate for your area, and the cost for each system both initial and over the long-term Snow and ice melting, propagation of cuttings and seed and the type of crop you will be growing are on the list of considerations as well.