Practical Snow Melt Design
The use of embedded hydronically heated tubing for the purpose of melting snow off of surfaces is a practical design application for plastic tubing. Such systems have successfully been utilized in walkways, parking lots, heliports, ramps, sports facilities, roofs and a host of other applications around the world. The desire for such systems arises out of several distinct benefits. They are: increased safety, reduced maintenance, improved operations, more pleasant surroundings, and as a method of discharging excess energy.
Initially, the thought of snow melting sounds straightforward and simple enough that no explanation seems necessary as to describe the intent of the system. Unfortunately, however, a true meeting of the minds between the designer and the customer cannot be satisfied by such a simple description of the product. Snow melting is complex and the performance varies greatly depending on the design of the system. It is wise and only ethical that the performance of the snow melt system meets the expectations of the customer.
When we refer to a depth of snowfall in inches, the measurement is meaningless for the purpose of design, or for gauging the performance of a system. Depth of snowfall does not take into account the density of snow (or the relative weight of snow with respect to water). Also, depth of snowfall doesn’t account for the temperature of the snow (although temperature is less important than density for calculating the amount of energy needed to melt snow). It is essentially the weight of the snow and not the depth that will determine how much energy will be required to cause it to melt. Just as it takes more energy to lift heavy snow when shoveling, it takes more energy to melt heavy snow with a snow melt system. Snow falls in the range of between 1% and 18% of the weight of water or at density of between .01 and .18. This is a fairly significant range, but a review of the National Oceanic Survey reports suggest that most snow falls around a density of .13 where 8 inches of snow melts into about 1 inch of water.
Raising the Temperature of Snow- In order to begin melting the snow, the temperature must first be brought up to 32 degrees F. The specific heat of ice is about .51 BTU/lb. Since snow is made up of ice, a pound of snow will require about 0.51 Btu to raise it one degree F. If 8 inches of snow weighs 5 pounds and is at 22 degrees F, Then it will take about 26 BTU per square foot to raise the snow itself to just below the melting point (32 degrees F).
Phase Change- The single most significant factor affecting the amount of energy required to melt snow is the energy required to change the water in snow from a solid to a liquid. This “phase change” is brought about by adding what is termed the “latent heat of fusion” to the mass. The latent heat of fusion for changing Ice to water is a whopping 144 BTU per pound. This being the case, Once our example of 5 pounds of snow has been raised to 32 degrees by the addition of 26 BTU per square foot, we will need another 750 BTU per square foot to actually melt the snow.
A typical problem in snow melt systems is under-sizing the delivery of energy to the point where the process of melting snow is “stalled” at the phase change. The result is the formation of slush and the transmission of heat into the atmosphere through radiation and convection at the upper surface of the slush. The energy transmitted to the atmosphere does not contribute to the objective of snow melting and is, therefore, wasted.
Snow Melt Theory: Much of the snow melt panel performance is either made or lost in the decisions regarding the snow melt plan. Knowing and understanding what is occurring during the process will enhance ones ability to make practical decisions about the design and to provide a product that will meet expectations. Within the broad category of snow melt panel performance there are at least two distinct considerations: the ability of the panel to melt snow, and the efficiency or cost in terms of energy that must be expended to accomplish the snow melt. Depending on the customers needs either may take higher priority over the other.
An old barnstormer pilot by the name of “Speed Holman” once said “...with a big enough engine I could fly a barn door”. Many of today’s snow melt designers have adopted old “Speed’s” attitude with respect to their system designs. Disregarding efficiency and the cavalier belief that large boilers overcome lack of engineering leaves the customer with huge energy costs and poor operational performance. Sadly, with just a little extra thought, significantly improved systems can be provided.
Some practical thoughts on Snow Melt Design: The energy that is put into the task of removing snow can be significant, expensive and , therefore, should not be wasted. Many designs actually work, but the cost of their operation is prohibitive. In such cases, a close analysis reveals that the reason they are expensive to operate is because much of the energy put into the system does not contribute to the task of eliminating snow. Such is often the case because the designer either opted for low initial construction or installation costs, or, worse yet, was ignorant of the way in which the system could be designed to operate at lower and more efficient energy levels.
Designers of snow melt systems traditionally obtain much of their experience from underfloor heating technology. Transplanting their knowledge and perceptions from such technology directly to snow melting systems leads to big mistakes. There are basically three reasons why this doesn’t work:
First, energy placed into space heating is somewhat captive. This is to say that areas of the building that may be overheated will share their excess energy with under-heated areas of the building and an inefficient design will not result in dramatic excess energy consumption. This is not the case with snow melt systems. Once the surface is free of snow, any further energy applied will escape freely into space without contributing to the task of melting adjacent areas. Enormous amounts of energy can wasted in this way and the designer must be aware of and avoid this situation.
Second, space heating systems are usually planar. Designers are not accustomed to dealing with three dimensional relief. They make their designs of flat paper and never seem to make the transition into using or accommodating the natural drainage of the surface to reduce energy consumption and improve performance.
Thirdly, space heating systems do not have to deal with phase changes. The relationship that energy has to performance is simple and linear. Energy lost is replaced by energy supplied and there are no “surprises”. The biggest single mistake made in snow melt design, particularly on large projects, is to view the project as a large monolithic slab and to attempt to swallow it whole with a simple control system and a massive boiler.
Consider the task of shoveling snow off a typical residential driveway with a thousand square feet of surface and a 12 inch snowfall weighing 3 pounds per square foot. Common sense tells you that with a two square foot snow shovel, if you can lift and throw 6 pounds adequately at a pace of ten scoops per minute, you can complete the job in 50 minutes. You will have lifted and thrown a ton and a half (3000 pounds) of snow and probably be tired. Using the typical snow melt designers approach to the task, he will calculate that you need a one thousand square foot shovel, and a device capable of lifting 3000 pounds all at once. At this point the task is beyond the capability of any human being.
The absurd case above is really no different than that which a snow melt designer faces when he ignores the problem of phase change. As the snow temperature begins to reach the melting point (32 degrees F), nearly three times as much energy will be needed, all at once, to transition the frozen water from a solid to a liquid. Whatever size area the designer chooses to melt (all at once) must be supplied with sufficient energy to accomplish this phase change in the time allotted, while accommodating all other losses to the atmosphere. If he wants to do it in an hour, he must multiply the weight of snow by 144 BTU to determine how much energy will be needed. A 10,000 square foot surface with 3 pounds per square foot snow load would require over 4.3 million BTU. This is why such large boilers are specified into snow melt systems. Is this a problem in design? Absolutely.
If the designer chooses to use a single large boiler (Speed Holman Concept) to do the project then he must contend with some significant events in the melting process that are likely to cause high energy consumption.
1. Since the surface is likely to be sloped towards a drainage system, melted snow and slush on the higher elevations will run off toward the lower elevations and the higher surfaces will dry off earlier. If the dry areas continue to be supplied with heat, then the energy will escape to the atmosphere without benefit. Soon, in order to eliminate snow from a very small remaining low lying area, more energy will be wasted than go to the intended purpose. This system will cost way too much money to operate than one more practically designed.
2. Also, since the amount of energy required to melt snow may be more than the system can deliver efficiently, the system may stall at the slush stage. Snow is a pretty good insulator (This is why igloos stay warm in the winter). When the surface is covered with snow, almost all of the energy that is applied from below goes toward melting the snow. As the snow begins to melt (phase change), the insulating effect is diminished until the upper surface becomes a form of slush. At the slush stage, some of the energy is transferred to the atmosphere from the water on the surface without contributing to the intended task of melting snow. This energy transfer becomes an additional load that must be supplied by the boiler, pumps and piping distribution system. If the air temperature is very cold and or even a mild wind is present (forced convection), this transfer can become very high and beyond the capability of the heat plant (Speed Holman stalls). If, however, the system is zoned to smaller surface areas, the amount of energy required can be limited to well within the capability of the boiler and distribution system, then sufficient energy can be supplied to the panel to “bust through the slush barrier”.
Using drainage to reduce operational cost.
Latent heat of evaporation. Once the snow has been heated to the melting point, the addition of energy will increase the temperature of a pound of water approximately one degree for every BTU added. If the water is allowed to pool, it will remain in the pool indefinitely unless it is allowed to evaporate. Evaporation is, of course, another phase change from the liquid to the gas. Just as additional energy is required to melt snow, even more energy is required to evaporate water. It is generally wiser to provide for drainage of the liquid water to a suitable storm sewer or holding pond. When underground, the earth’s mass will prevent re-freezing of the run off. It is important, while designing the system, to provide a heated pathway of drainage for the run off water and, in some cases, to provide additional drainage to control the movement of run off past already cleared areas.
A run off plan. The water shed should be planned in advance. It is important to realize that a rain run off plan may not be suitable for snow melt operations due to the damming effect of snow and ice.
Practical efficiency: When one reviews the monthly and annual snowfall figures, it is surprising how little snow actually falls over much of the earth and how large a problem it can be. The amount of energy expended should be very little for most of the “civilized” world, but the combined efforts of road crews, private snow removal services, and individual property owners amount to an enormous cost. The additional cost of lost business and efficiency can hardly be estimated. One can’t help but be amused by the city snow “removal” plows that pile the snow on the sidewalks where the property owner blows it back onto the streets. The process sometimes seems to go on until the snow is literally beat into nothingness. It always seems that the areas with the most need for snow removal have nowhere to put it. Snow melt systems, on the other hand, don’t just move the snow, they remove it. Done effieciently, there is probably no more practical manner of dealing with snow. Done inefficiently, there is probably no more expensive way of dealing with snow.
In summary: a practical approach to snow melt design is to:
vDefine (in terms of equivalent water weight) the amount of snow that must be removed per hour (or other convenient time frame).
vDetermine the lowest outdoor design temperature at which the system will operate during snow melt phase.
vCalculate the energy requirement (Btu/hr) necessary to accomplish snow melting at the design conditions.
vDesign the run off plan to insure that meltwater is not allowed to accumulate on the slab.
* Prioritize areas in large slabs. Give priority to meltwater run off channels and any specific operational needs (such as pedestrian entry walkways, VIP parking, loading docks etc.).
* Zone the system panel sizes that are well within the capability of the heat plant’s ability to blast through the slush stage.
* Calculate the total system energy requirement.
* Select and design a control strategy (on / off, idling or combination).
* Select and design the zone control, distribution, heat plant and tempering system.
Copyrite Tesmar Application Technology 1995