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This is a short video tutorial designed to help you learn how to calculate the heat
requirements for a greenhouse. Whenever designing a greenhouse it's important that you size
your greenhouse heating system appropriately so that it will meet the temperature
requirements for your crop.
In this problem set we are using the example of a glass covered greenhouse with a metal frame
measuring 30 feet wide by one hundred feet long The greenhouse has a curtain wall,
which is 2 feet high and is constructed with four- inch concrete block. The glass wall
above the curtain wall is 6 feet high. The site location has an average wind velocity of 15 mph and a 60° temperature difference between the outside low temperature of zero and inside temperature of 60°F . This problem set is taken directly from Dr. Paul Nelson's textbook Greenhouse Operation and Management, 7th edition and can be found in its entirety on pages 112 through 114.
Here we have a typical layout of a greenhouse as described in your textbook on page 109. We have four main components with this greenhouse. The gable, which is the triangle on the top of the end wall. Four greenhouse walls of course and the greenhouse roof, which has two sides. The curtain walls are typically designed for a greenhouse at the height of the benches. The main philosophy of a curtain wall is to cut down energy loss by using a more efficient construction material for the curtain wall without interfering with any light transmission.
The example greenhouse is 30 feet wide.
And 100 feet long. This is a standard greenhouse size that you will find in most commercial construction settings. Greenhouses will vary in width and size based upon your own designs and specifications as well as the standard sizes that most greenhouse manufacturers choose.
This greenhouse example has a wall height of 6 feet or 6 feet above the curtain wall. This is the part of the wall that is transparent to light.
And the curtain wall for this example is 2 feet high. Many modern greenhouses are actually taller than 8 feet, but when the original version of Nelson’s textbook was written a 6 foot wall plus a 2 foot curtain wall was the typical greenhouse wall height. Some greenhouses are designed to be 2 feet into the ground to eliminate the need for curtain wall and providing additional insulation value, which makes the greenhouse more energy efficient.
To begin the calculations, we need to set up a table that will help us to develop the heat loss calculations for this greenhouse under the conditions in the example used in the textbook. It's much the same as balancing a checkbook. This example uses simple arithmetic and is not very hard to do, but it does require some understanding of how the values are determined, which values to use, and where you can find those to include in this table.
The first set of values that we need to determine are the standard heat loss numbers for each greenhouse component. The standard heat loss assumes a uniform heat transfer across the surface area of the greenhouse component and we determine this in MBTUs per hour or thousand BTUs per hour. Remember that a BTU is a British thermal unit and this is the amount of energy crosses through a material illustrating how much energy is lost per unit hour.
Let's go back to the drawing of our greenhouse. The first thing that we need to do is calculate the energy loss of the gable section of the greenhouse. The gable section is the triangle that is pictured on top of the wall.
This example assumes that the roof is a six by 12 pitch. To understand this, divide the gable into two right triangles
with the height of the triangle equal to six units and the base 12 units. The hypotenuse of the triangle is then at a constant fixed ratio. (remember the Pythagorean theorem?) This allows us to use a standard table of values.
The standard table that's used in Nelson's textbook, table 3 dash 7, is adopted from the Acme Engineering and Manufacturing Company Climate Control Handbook. Acme designs and sells heating and cooling equipment for greenhouses and agricultural buildings. You can find this handbook in the supplemental reading materials on the class website in RamCT. This table is somewhat confusing in that it has both the values for the gable and the roof are on the same table. Remember that this table assumes that your gable and roof configuration is a 6 x 12 pitch. We will use the top section of this table, where you find your greenhouse width. The row of numbers under “Gable Loss” is the heat loss in thousand BTUs per hour. The parentheses are metric values.
The small number 2 is not a square value, but a footnote indicator.
Our example greenhouse is 30 feet wide
As indicated by the red circle
And the heat loss value for the both gables at either end of the greenhouse is 18,000 BTUs per hour.
As indicated by the lower red circle. When using these tables it is important to remember the heat loss values are for both ends of the greenhouse. If you are designing a greenhouse that is up against a head house with only one gable exposed to the outside, then you divide this number by two.
And here we enter the number 18 in the standard heat loss column
Next, we need determine the heat loss requirements for the roof. Using the tables from the textbook we only need to know the width and the length of the greenhouse. Remember that this assumes a 6 x 12 pitch.
So you go back to the table and find the greenhouse width
Which is 30 feet
And now we use the lower part of this table and find the roof length
Which is 100 feet
And again the values in parentheses are the metric units
We follow down from 30 feet and across from 100 feet
And then find the value 266,000 BTUs per hour
Now place that number in the standard heat loss column for the roof value. Again, remember that this is for both sides or pitches of the roof.
Next we need to determine the heat loss for the greenhouse walls, which are 6 feet high
The length of the walls is equal to the perimeter of the greenhouse.
The two walls ends, 30 feet wide times 2 and the two sidewalls 100 feet long times 2 equals a total perimeter value of 260 feet
Using the next table from your textbook, table 3 dash 8, which again is from the Acme engineering and Manufacturing Company, we first find the wall height, which is 6 feet
And then the wall length or perimeter
However if you look at the table, 260 feet doesn't appear, so what do we now do?
The answer is that we need to split the 260 feet into parts, which are on the table. The easiest thing to do is to first take the value for 200 feet
And then the value for 60 feet
Which results in 95,000 BTUs per hour for the 200 feet section of wall and 28,000 BTUs per hour the 60 foot section of the wall
Which results in 95,000 BTUs per hour for the 200 feet section of wall and 28,000 BTUs per hour the 60 foot section of the wall
And adding 95 and 28 results in a sum of 123,000 BTUs per hour for the greenhouse walls
And then place the value of 123,000 BTUs per hour into the standard heat loss column of our table
Next we need to calculate the heat loss of curtain wall curtain wall and in this
Next we need to calculate the heat loss of curtain wall curtain wall and in this
example, the curtain wall is 2 feet high
So now we use the 2 foot high wall height column. The perimeter calculations are
still the same.
Which gives us heat loss values of 32 and 9, which when added together results in 41,000 BTU
hours for the two foot-high curtain wall
And then place the value 41,000 BTUs per hour in the standard heat loss column
The next step in our calculations is to determine what is called the K-value. The
K-value is a reference to the climate where the greenhouse is located. It is important that
you size your greenhouse heating system for your location. The same greenhouse
design located in the southern United States compared to one in the northern United States will
require different heating capacity. You also need to take into account the wind speed of
the area and the site. As the wind speed increases so also increases the heat loss from
your greenhouse
So in this example we will assume an inside greenhouse temperature to be maintained at
60°F
And in this example, we will assume an outside greenhouse temperature 0°F. One thing that
a lot of people forget to consider is that when building greenhouses you must use the
typical low temperatures for greenhouse location during the winter. Using average
temperatures is typically a mistake as the numbers are misleading. It is best to
consider a worse case in our scenario for low temperatures in order for your greenhouse
heating system to maintain adequate production temperatures.
And this gives us a heat loss value because energy goes from high to low, which means that
heat energy moves from inside the greenhouse to outside the greenhouse. The difference
between these values is going to be 60°, 60°F
And we are going to assume a wind speed of 15 mph, but if you get strong continuous gusts
of higher values, you will probably need to think about using a greater value
And as the wind blows across the roof of your greenhouse, it is going to increase the rate that
energy is moved from within the greenhouse
Our example greenhouse location assumes a wind speed of 15 mph and we can find this value on
the table from Nelson’s textbook, table 3 dash 9. Again this table is from Acme
Engineering
the delta T
Which yields a K value of 0.84. These are engineering units that are used to determine how fast
energy is removed from the greenhouse. It was determined by engineers back in the 1960s
and 70s from a group of agricultural engineers in the northeast Region of the United
States. Think of these values as correction factors without units
So we put the value of 0.84 in the K value column. The number is the same for all or
greenhouse components: the gable, roof, walls, and curtain walls
The next value that we need to include is called the C value or the construction value. The
CW is value is for the curtain wall value and not to be confused with C value
So to determine the C value, we need to know what the components are constructed from. Each part
of our greenhouse in this particular example assumes a metal frame greenhouse with a
glass roof and walls. You might have fiberglass, polycarbonate or you may have different parts
on the roof and different parts of the wall from different construction materials. One
question that I often get from students completing this assignment is, are the gables
and the wall directly below the gable the same? In most instances the gable and the
wall directly below typically made of the same construction material
wall directly below typically made of the same construction material
wall directly below typically made of the same construction material
wall directly below typically made of the same construction material
wall directly below typically made of the same construction material
wall directly below typically made of the same construction material
wall directly below typically made of the same construction material
wall directly below typically made of the same construction material