Energy Savings
|
Info Pages
|
|
Energy Savings in Drying by Paul Tedman, PE
An important key to efficient drying is reducing the energy that is
exhausted. This can be done by properly sizing the dryer and the
exhaust system. Many dryers can be improved by tuning the exhaust
system for the processing conditions. Return
on investment
At
the same time, little attention has been given to drying efficiency.
For almost two decades now, we have had relatively low fuel prices
in the US. However, with near quadruple increases in the price of
natural gas in some geographic areas, the winter of 2000-2001 has
reminded many of the US energy crisis of the early 1980s. The majority of the dryers in the US used in conjunction with petfood
extrusion processing are heated with natural gas. As the cost of
natural gas approaches US$10 per 1,000 cubic feet, the annual cost
for the gas will reach nearly US$250,000. In
recent years, when processors were paying US$2.50-3.00 per 1,000
cubic feet, there were not many savings if the efficiency was
improved by 10 to 20%. However, as the price of natural gas
increases, the savings become more significant. Based on the data in
Figure 1, a 10% improvement in gas consumption can save the
processor nearly US$25,000 annually as the price approaches US$10
per 1,000 cubic feet. While
gas prices have subsided somewhat since the cold winter months of
2000-2001, the prices are still higher than what we have become
accustomed to in recent years. Many processors are now looking to
improve the efficiency of their drying operations in order to lower
their operating costs. Evaluating
efficiency
The
energy that enters the dryer must equal the energy that exits the
dryer. Therefore, the following can be derived: QiP
+ QiB + QiMU = QoE + QoC + QoP
Or QiB
= QoE + QoC + QoP – QiP – QiMU Energy
inputs
The
amount of energy that enters the system from the make-up air (QiMU)
is primarily dependent on the ambient conditions and the operator
has little control of this variable. As the ambient air temperature
increases, less energy is needed from the burners (QiB). As the
ambient air temperature decreases, more energy will be required from
the burners. Energy
outputs
Convective
energy losses (QoC) occur in all dryers, although most dryers are
insulated very well. The convective losses are greater for dryers in
cold ambient conditions compared to those in warm ambient
conditions. However, overall convective losses are usually not
significant when compared to the other losses. An
energy loss also occurs as the product exits the dryer (QoP). This
loss depends on the temperature of the product and its moisture
content. Fortunately, this loss is less than the energy gain from
the hot, moist product that enters the dryer. The reason for this is
that the heat of evaporation temperature is much less than the
temperature of the product as it enters the dryer. In addition, a
large portion of the moisture is removed from the product in the
dryer. Integral cooling devices will help further reduce the energy
loss from the product, but these devices are not being utilized as
much as they were in the past due to other processing
considerations. The energy loss out of the exhaust stack is by far
the most significant energy loss in the system. Referring back to
the heat balance equation (QiP + QiB + QiMU = QoE + QoC + QoP), it
can be seen that excessive heat loss from the exhaust duct must be
equalized on the left side of the equation by an energy gain (See
Figure 3). The only variable that can make up the difference is QiB,
which is energy input from the burners. Efficiency
Theoretical
energy requirements can be obtained from a saturated steam table. At
an atmospheric pressure of 14.696 pounds-per-square inch (PSI), the
total required energy to evaporate one lb. of water is 1150 BTU.
This includes both the sensible heat and the latent heat of
vaporization. In the extrusion process, the product enters the dryer
at elevated temperatures, sometimes as hot as 200ºF. This means
that a significant amount of the sensible heat remains in the
product. Therefore,
it is probably more appropriate to use only the latent heat of
vaporization in the efficiency calculation, which would be 970
BTU/lb. The true efficiency can be calculated by dividing the latent
heat of vaporization (970 BTU/lb.) by the actual energy needed to
evaporate one lb. of water. So if it takes 1200 BTU/lb., then the
true efficiency would be (970/1200) X 100% = 81%. At 1500 BTU/lb.,
the efficiency would be 65%. Improving
efficiency
The
amount of air to be exhausted depends on the temperature of the
exhaust air. Exhaust air temperatures typically range from 140-190
ºF for most petfood dryers. The amount of water vapor that can be
carried by the air at these temperatures also varies. Efficient
dryers will exhaust air that is moderately saturated. Completely
saturated air can create condensation problems along the ceiling and
in the ductwork. As
the temperature increases from 140 to 190 ºF, the specific humidity
increases from .060 to 0.129 lb. of water/lb. of dry air. This
simply means that less air needs to be exhausted at higher
temperatures, which in turn, improves efficiency (See Figure 5).
Many dryers have been oversized in the past to ensure that there
will be sufficient retention time to dry the product. If the dryer
is larger than necessary, then it is likely that the unit is
operated at low temperatures. This will result in low exhaust
temperatures, and in turn, the efficiency will be poor. Some operations have multiple extruders feeding a single dryer. At
times, only one extruder may be in operation, which essentially
results in an oversized, inefficient drying operation. In addition,
some extrusion operations today process a multitude of different
products. A single line may produce some products at 4-ton/hr and
produce other products at 8-ton/hr. In addition, the wet and dry
moisture contents may vary significantly. These types of processing
conditions make it difficult to have efficient drying operations.
Those processors that produce products that are very similar will
likely have more success in improving efficiency than those that
process a wide range of products. Overall, the key to an efficient drying process is to reduce the major energy loss as much as possible, which is the energy that is exhausted out of the stack (QoE). This can be done by properly sizing the dryer and the exhaust system. Many dryers in operation today can be improved by simply evaluating the exhaust system and tuning it for the processing conditions. Careful consideration should be given to all processing variables when planning a new installation. It is important not to undersize a dryer from a production rate standpoint. However, from an efficiency standpoint, it is critical not to oversize the dryer |