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 Perhaps
the most common piece of equipment in commercial greenhouses...
...is the gas-fired unit heater.
These
generic-looking devices are hanging around in great numbers...and
almost every greenhouse operation has them.
Of course many operations are heated with the more efficient
hot water boiler systems, but gas-fired unit heaters are
by far the most common heating appliance you will find
in greenhouses in the United States. Although, this is
an article about a specific product, it’s important
to understand as much as possible about the world of unit
heaters. This is especially true now that the robber-barons
are back running the fuel markets!
Little boxes that roar!
Based on the 1999 USDA Floriculture Report there are
533 million square feet of enclosed production greenhouse
space
in this country, and approximately
70% of it is heated with forced air. With the average grower spending
around $.60 per square
foot on average annually to heat, we’re talking about an annual throughput
of fuel dollars of around $223. million dollars! That’s almost 1/4
billion (with a “B”) dollars being used annually by unit heaters
in our industry!
Wow. If they use up so much revenue, how come they seem
so. well, innocuous? The answer lies somewhere in the
habit of many American growers to not look at the long term when
investing in heating equipment and concentrating mostly
on initial costs. “Fuel is cheap, why do I need efficient
heaters?”, they said. Many growers do not understand
the huge differences in efficiency between the various
types of unit heaters on the market. However, with the
upswing in fuel prices, it’s a good bet that growers
will do a bit more research when making purchasing decisions
from here on out.
Understanding the differences:
Ok, you have a metal box, you put gas to it, it burns,
and a fan disperses it into the greenhouse space. What’s
the big deal?
 Direct Fired Units: No heat exchanger
Direct-fired unit heaters have a burner, a flame, and
a fan. They are extremely simple. They operate thermostatically
by running the burner and blowing air
(sometimes a mixture of inside and outside air) over, around, and through
the flame, picking up heat along the way. These units send
this heated air and
products of combustion into the growing space. This is a mixture of carbon-dioxide,
carbon-monoxide, and ethylene— with some traces of sulfur and a few
other things that may not be desired in your plant production space.
Still, they are popular in some types of production, like retractable greenhouses
that may not be very tight, and some cut flower operations growing blooms that
are not sensitive to the ill-effects of ethylene or too much CO2. Note:
It’s really important to keep the safety
of your staff in mind when considering a heater with no heat exchanger,
as
carbon-monoxide is extremely poisonous
to humans.
Heat Exchanger Equipped Units:
Most growing operations must install units that separate the products of combustion
from the distributed warm air with a heat exchanger. This is because we cannot
afford to fill up our greenhouses with the toxic elements previously mentioned
for fear of damaging our crops or endangering our staff.
A unit heater’s heat exchanger is nothing more than a metal barrier designed
to separate hot combustion gases from greenhouse air. The metal surface of
the heat exchanger transmits the heat of combustion efficiently to it’s
surface. The unit heater’s integral fan then transfers the heat to the
air.
A heat exchanger for a greenhouse unit heater must have
plenty of surface area and be optimized for survival in
a humid, jungle-like environment. Many crops have been
lost,delayed, or damaged by unit heaters with leaky heat
exchangers. Currently, there are two basic heat exchanger
designs on the market, stamped sheet metal “clam-shell” designs,
and seamless tubular “serpentine” designs.
The latter has been offered for approximately 6 years,
while the clam-shell design has been around many decades.
So, now we’ve got a hot piece of metal that we’re blowing air over— that’s
the basic design of all the heaters you see in greenhouses everywhere. What
are the areas that make heaters different from one another? How can one heater
of the same BTUH size use less fuel annually than another? Why shouldn’t
I just buy the cheapest one? After all, they’re just painted metal boxes,
aren’t they?
So many questions— no wonder many growers just
opt for the cheapest “250” they can get— at
least prices are a solid comparison. Right? Well, not exactly.
Startling Truths Revealed!
There are huge differences between heaters being sold today. If every grower
using unit heaters immediately upgraded to better technology, we’d slash
that $1/2 billion (with a “B”) by as much as 25%! That’s
a cool $60 million a year that could go right to the bottom line! That’s
a big number to our industry, but each grower needs to weigh the economics
and Return On Investment (ROI) for their own facility. It’s very common
to see a ROI of less than 2 years when fresh upgraded unit heaters are placed
in a facility that had been operating with older units. The smart money is
being spent on heaters with the following features.
Induced Draft:
Simple as it sounds, adding a little fan on the exhaust of a unit heater and
installing the chimney out the end of the house and pointing the termination
down (please follow each manufacturers recommendations for proper venting),
can eliminate 10-15% of fuel use. The reason is explained with simple physics.
We all know that hot air wants to go up, right? If your unit heater is of the “gravity
stack”variety— and most of them are, every time your thermostat
turns off, all the hot air you just generated starts looking for the “easy
way out”.
Guess what? It’s the chimney stack of your heater
that is most likely terminated above the ridge of the house
so, as soon as you make the heat, it starts leaving out
the very same place it was introduced. Does this make any
sense at all? No, of course not... but unless you are aware,
you’d never know it unless you had infra-red scanners
outside your greenhouse.
With an active (induced) venting system, when the heater
stops firing, the venting system is one of the last places
your precious heat will want to go because it’s pointed
down. These little fans are sized to positively evacuate
the products of combustion out of the heat exchanger. They
do use an inconsequential amount of electricity, but you
won’t notice the cost. The addition of an induced
draft fan increases the cost of the heater, but it almost
always decreases the cost of venting... and the fuel savings
begin piling up.
Outside Combustion Air:
So now we understand a bit more about venting the products of combustion, but
let’s look a little closer at the combustion process. Fuel and air must
mix for combustion to be generated. This air is
typically obtained from the
surrounding air around where the unit is located. That’s right, from
inside the green house.
Let’s think about that for a while. We’re using heated air from
inside the greenhouse to make more heat. And, greenhouse air can be very
humid, and can be laden with chemical particulates. This does not seem very
logical. Not only are we using what is probably
the poorest quality air for combustion, which will reduce
the life of the heat exchanger, but we’re also ejecting
already heated air from the greenhouse through the chimney.
Do the math and you’ll find out that this in itself
is a giant loss of efficiency.
A typical induced draft 250,000 BTUH heater has an exhaust
fan that runs at about 60 CFM, or 3600 cubic feet per hour
(CFH). This is exhaust comprised of already heated air
and the by-products of combusting fuel. A typical bay of
a typical greenhouse (30’ x 96’ gutter house)
has approximately 36,000 cubic feet of air volume. Every
hour that your heater runs, 10% of the heated inside air
is ejected. This means that your heater has to “re-heat” 10%
or more extra make-up air every hour... that’s expensive
and wasteful.
So, what’s the solution? Bring the combustion air
to the unit heater in a separate duct from the outside.
This way, you are heating only re-circulated greenhouse
air and not being forced into “compulsory” venting.
This also eliminates the problem of burning the tainted
inside air. Burning cool, oxygen-rich outside air will
make your unit heaters happy for years. Separated combustion
is available on the market, but in most cases it exacts
a hefty price penalty and a slower ROI which brings us
to the featured product of this article.
 Lennox-LF-24 heater with CAS technology:
The Lennox LF-24 series of heaters are based on a “clean-sheet” design.
The engineers tossed out all the paradigms about what a unit heater is supposed
to be and looked at the process.
Heat Exchanger:
The engineers looked at the problems with traditional heat exchangers... cracking
at welds, metal fatigue from being stamped, and corrosion attacking welds and
they decided on a different course. The heat exchanger design they settled
on has been wildly successful. It is a simple design using a series of cold-formed
aluminized steel or stainless steel tubes expanded into sheet metal end plates.
The design is completely seamless which eliminates the problems associated
with welded and crimped joints failing prematurely. These heat exchangers are
designed to last... and last.
“In-shot” Burners:
The combustion process occurs over most of the length of the heat exchanger
tubes. Fuel is mixed with combustion air and introduced through multi-ported
disc-like burners positioned at the inlet to the tubes. Drawing the mixture
and creating very complete combustion is the induced draft fan and internal
turbulators that swirl the flame thereby extracting maximum energy from combustion.
These burners are more akin to a fire-tube boiler burner than the normal unit
heater with “atmospheric” combustion.
Ignition System:
Instead of building a system on top of another system like other manufacturers,
the engineers went with a “direct-spark” system that lights the
burners directly, rather than lighting a pilot first, and then the burners.
Induced Draft:
A small squirrel-cage fan is located on the outlet of the heat exchanger and
the products of combustion are drawn through the heat exchanger and exhausted
through the induced draft fan. This facilitates simplified and much more efficient
venting as described earlier.
Internal Diagnostic System:
This innovative system provides a very simple means of trouble-shooting. In
the event of a failure, sequencing flashes from green LED’s communicate
exactly the source of the problem, thereby expediting the repair process. Modular
components are easy to remove and replace.
High CFM Distribution Fans:
The volume of air moved and the throw distance of effective heating are more
than the traditional units on the market. The features are extensive and the
enhancement of CAS technology makes it an ideal choice for greenhouse applications.
CAS stands for Combustion Air Separation. It’s an enhancement to the
basic heater that provides a very cost effective solution optimized for greenhouse
applications.
The Lennox LF-24 series of heaters are a modular approach to heating. They
are unlike any other heater being sold. They can be purchased as a standard
induced draft heater, and upgraded with CAS to have access to outside combustion
air and all the good that that entails. This modular approach delivers a much
higher value than other heaters on the market.
Other features available for the LF-24 series are Poly
tube inflation adapters, blower-style fanupgrades, and
aluminized steel or stainless steel heat exchangers. Also,
growers that operate facilities at high elevations will
be glad to know that no de-rating is required on these
units until you get above 7,500 feet above sea level.
The LF-24 series unit heater enhanced with TrueLeaf’s
CAS technology delivers a lot of bang for the buck in an
efficient, modular package.
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