Ef = mi (Li + ci
T)by
Ice accumulations on cables, gears, steel plates, and concrete walls on lock and dam machinery can hamper or even halt project operation. Removal of this ice can be hazardous and time-consuming. In the past, removal has been accomplished mechanically by chipping or thermally by melting with hot water or steam.
More recently, various heating devices have been placed in critical areas to prevent ice formation or to melt existing ice. These devices include heated panels, bubbler systems, radiant heaters, and cartridge heaters. Recently, the performance and applicability of portable space heaters for melting ice were investigated. These heaters have been used successfully at Peoria Lock and Dam on the Illinois Waterway to melt ice accumulations from the bull gear pit. They range in size from 20,000 to 400,000 BTU/ hr (6 to 120 kW/hr) and can be fueled by propane, oil, or kerosene.
Under the REMR Research Program, the U.S. Army Cold Regions Research and Engineering Laboratory (CRREL) has evaluated the performance of space heaters for melting ice. The purpose of these tests was to determine the effects of air temperature, wind speed, and distance between the outlet and ice surface (standoff) with the use of hot air to melt ice. The test setup is shown in Figure 1.
The tests were conducted outdoors, and a fan provided the desired wind speed. The ice blocks were 2 ft (0.61 m) square and about 3 in. (76 mm) thick. Each block was placed on a wood frame that was suspended by two load cells. The hot air was provided by a propane-fired 150,000 BTU/hr (44 kW/hr) Universal TM heater (model no. 150- FAS). For 12 of the test conditions, the blocks were placed flat, and the hot air was delivered from the outlet of the space heater to the horizontal ice surface via an insulated metal duct, as shown in Figure 2. For the remaining four test conditions, the ice was tilted on an incline ranging from 30 to 80 deg from horizontal. For these tests, the duct was removed, and the outlet of the heater impinged directly on the ice block. Ambient air temperature, duct outlet temperature, and ice surface temperature were measured throughout the tests. A typical test lasted 30 min to 1 hr.
The performance parameter calculated for the heater was melting efficiency,
e
e = Em/Ef
where Em
is the minimum amount of energy required to melt the ice and Ef
is the energy expended melting the ice. Thus, Ef is
calculated by multiplying the mass of propane consumed during the test, mp,
times the heating value of propane, hp.
Ef
= mphp
Similarly,
Ef = mi (Li
+ ciT)
where mi is the mass of ice melted, Li
is the latent heat of fusion for ice (333 kJ/kg), ci is the
specific heat for ice (2.04 kJ/kg-K), and T
is the difference in temperature between the ice block and the freezing point at
the start of the test.
The results of the horizontal surface tests are presented in Table 1, and the results for the inclined tests are presented in Table 2. For cases where the number of tests is greater than one, the standard deviation is also calculated.
The results of these tests showed that, over the temperature ranges tested,
ambient air temperature has little effect on the melting efficiency of the space
heater (Figure 3). This finding is not
surprising because the outlet temperature was typically 400° F (200°
C) while the air temperature was between 14° and 41° F (-10° and
5° C). The amount of heat transfer is driven mainly by the temperature
differential between the melting temperature of ice and fluid (in this case the
exhaust gases); therefore, fluctuations in T
of 9° to 18° F (-13° to -8° C) at the most were only about 2
percent of the temperature difference between the heater outlet temperature and
the melting temperature of ice. Thus, the temperature of the exhaust gases
dominates the heat transfer, and the air temperature primarily affects only the
sensible heat stored in the ice block, which is typically very small in
comparison to the latent heat of ice. For example, with an air temperature
(hence initial block temperature) of 14° F (-10° C), the sensible heat
is only about 20 J/g, or about 5 percent of the latent heat of fusion for ice.
Even if the ice temperature were to drop to -10° F (-23° C) (an air
temperature frequently seen at many Corps projects in the northern part of the
United States), the sensible heat represents less than 15 percent of the latent
heat of fusion for ice. Thus, the heat required to melt the ice dominates for
all air temperatures of interest in this problem.
We also found that under no-wind conditions, the standoff distance has virtually no effect on the melting efficiency (for distances ranging from 2 to 12 in. (51 to 205 mm)), which remains nearly constant at 4 to 5 percent. However, standoff distance does play an important role in the presence of even moderate winds. Figure 4 shows the melting efficiency for standoff distances of 3 and 6 in. (76 and 152 mm) with no wind and with a 7-mph (11-km) wind, respectively. In the no-wind case, the two standoff distances perform almost identically. In the presence of a 7-mph wind with a standoff of 3 in., there is a moderate decline in efficiency of about 25 percent. Yet if the standoff distance is doubled from 3 to 6 in., the efficiency declines by 75 percent.
Indeed, eliminating the effects of wind plays a major role in the efficient melting of ice with space heaters. Figure 5 compares the drop in efficiency with wind speed for air temperatures of 28° and 14° F (-2° and -10° C). In both cases, we can see the wind cuts efficiency significantly. Interestingly, the slopes of both lines are almost the same, and the average slope for the two lines is -0.006/mph (-0.01 km/hr) over the wind speeds considered in this study. This is about a 12-percent loss in melting efficiency for an increase of 1 mph (1.6 km/hr) in wind speed.
In the inclined ice tests, we found that the angle of impingement had no effect on the melting efficiency. In fact, the only real difference we witnessed was an approximate 15- to 20-percent increase in overall efficiency compared to the horizontal tests. We attribute this change to removing the duct, thereby recovering the losses associated with ducting the hot exhaust gases (i.e., radiation losses from the duct).
In general, we find that melting ice with hot air is a very inefficient process, with not much more than 5 percent of the energy stored in the fuel going to melting the ice. Tests conducted at CRREL using the exhaust gases of a gas turbine engine for melting ice yielded similar results with maximum efficiencies never exceeding 8 percent. Since modern combustion chambers are highly efficient, yielding fuel conversion efficiencies on the order of 85 percent or more, we attribute no more than 15 percent of the loss of energy to incomplete combustion. This means nearly 80 percent of the fuel energy is lost through heat transfer effects such as heat losses through the heater housing and duct work. In addition, incomplete heat transfer between the hot air and ice surface reduces melting efficiency. These tests were conducted in an open-air environment. There was nothing to prevent the hot air from leaving the proximity of the ice surface after it exited the outlet, so most of the heat was carried away in the hot air with very little heat being transferred to the ice surface. These losses can likely be reduced by enclosing the heated space with plastic (Figure 6), which would eliminate wind losses as well as raise the ambient air temperature.
Portable space heaters are readily available at most Corps projects. This work shows that they can be used to melt ice, though under the best of circumstances they have melting efficiencies of only about 5 percent. Wind and losses due to free convection severely reduce the efficiency of melting ice with hot air. A simple means of reducing these effects is to enclose the area to be deiced within a shelter. If it is intended to be a temporary structure, plastic over a wood frame would suffice. Because of the low ice- melting efficiency of space heaters, this method of deicing or ice prevention should be seen only as a stop gap measure, and more efficient deicing methods, such as heater panels or bubblers, should be used as permanent solutions to perennial icing problems.
For additional information, contact Robert Haehnel at 603-646-4325.