Designing Snowmelting Systems, Part 2-Piping and Controls
Main Mechanical ComponentsHeat Source
A variety of heat sources may be used to heat the fluid. Heat sources may utilize steam, hot water, gas, oil or electricity. Alternative heat sources may utilize engine generators, condensate and other waste heat sources. Renewable energy resources may be used with heat pumps. Consideration should be given to flue gas condensation and thermal shock to the boiler from low return fluid temperatures. Bypass lines and four-way valves may be necessary to maintain the recommended boiler temperatures. Condensing boilers are a good option because they are made to operate at low temperatures. If a condensing boiler is used for a dedicated snowmelting system, a mixing device may not be necessary. With a condensing boiler, the lower the operating temperature, the higher the efficiency will be.
There are several ways to accomplish water temperature control. One such method is to use a mixing valve. An added advantage of this is the ability to control return water temperature to the boiler. In an economical system, water temperature control may also be accomplished with a three-way diverting valve. Some snowmelting systems are piped with injection systems.
To isolate the snowmelting fluid from the rest of the system, heat exchangers are usually suggested. By isolating the system, the glycol can be more easily controlled and less can be used. Selection involves both thermal and hydraulic criteria. Thermal performance has to do with the transfer of heat from one liquid to the other at a specific rate, given the entering temperatures and flow rates. Hydraulic issues relate to head loss and pressure drop. The heat exchanger provides isolation of the glycol from the boiler water, providing an oxygen barrier to the boiler and protecting other components in the boiler system (with a nonferrous heat exchanger). This allows non-barrier PEX pipe to be used in snowmelting systems if needed. There are several computer software programs available to assist in sizing heat exchangers specifically for snowmelting systems. Most manufacturers have technical support departments that will provide heat exchanger design assistance if requested.
To accommodate the volume increase of the fluid as it is heated, an expansion tank is used. The system must be equipped with one on either side of the heat exchanger (one per closed system). There are three basic tank configurations: a closed tank which contains a certain volume of compressed air (e.g., a plain steel tank); an open tank (i.e., open to the atmosphere); or a diaphragm tank, in which a flexible membrane is placed between the air and the water (i.e., a bladder tank). Current design practice normally suggests diaphragm tanks, because gases can enter the system water and adversely affect system performance with the closed tank and open tank. A glycol solution expansion rate is approximately 1.2 times greater than that of water. Thus, the tank should be at least approximately 1.2 times larger than for a corresponding water-filled system.
Since the expansion tank always maintains the same fluid pressure at the point it is connected to the system, the preferred location of the circulator is pumping away from the expansion tank. This way when the circulator is turned on, the pressure increases in most parts of the circuit, which helps eject air from vents. In snowmelting systems that utilize a heat exchanger and non-barrier PEX tubing, cast iron circulators should not be used because of corrosion. In such applications, bronze or stainless steel circulators are recommended.
Manifolds are often referred to as the distribution center. Typically 1-1/2" in size for snowmelting purposes, good practice is to equip the manifolds with flow meters and isolation valves. This will help with isolating circuits for filling and purging. The flow meters will allow for easy balancing if circuit lengths are not all equal (Figure 1).
Controlling the Snowmelting SystemAn automatic snowmelting control is essential to proper system operation and efficiency. For example, if the snowmelting system does not activate until the snow starts falling heavily, it may not melt the snow effectively for hours, thus giving additional snowfall a chance to accumulate and increasing the amount of time it takes to melt the area. An automatic control will activate the snowmelting system when light snow starts (with the proper ambient temperature), thus allowing ample warm-up before heavy snowfall develops.
In selecting a control package, the designer must define what the expectations of the snowmelting system are. Following the concept of the basic and advanced piping arrangements, two types of control packages are used, for the most part.
A control package that provides automatic system activation with slab high limit is recommended even in applications where cost is a concern or where the snow sensor cannot be mounted in the thermal mass. A snow detector control senses air temperature and falling snow to activate the snowmelting system. A slab sensor prevents overheating of the slab. A timer switch allows for optional manual activation. These components can easily be integrated to provide a basic snowmelting control package (Figure 2).
An advanced snowmelting control dramatically increases the sensitivity of the system, thus increasing the operation efficiency of the system. Typically a microprocessor-based control equipped with an in-slab temperature and moisture sensor (Figure 3), an advanced snowmelt control has numerous practical and valuable features which may include slab protection, viscosity compensation, boiler protection and idling capability.
The slab protection feature allows the control to limit the rate at which heat is applied to the snowmelting area by limiting the ?T between the snowmelting supply temperature and the snowmelting return water temperature. By limiting this temperature difference, the rate at which heat is applied to the zone can be controlled, and thermal stresses in the slab can be minimized.
Viscosity compensation is important at low temperatures when the glycol solution becomes very viscous and difficult to pump. In order to overcome this condition during a cold start, the glycol solution temperature must be increased. The viscosity compensation feature allows the maximum ?T between the snowmelting supply temperature and the snowmelting return water temperature to be increased for a period of time. Typically this is used when the return temperature is below 30?F.
The boiler protection feature protects the boiler from encountering cold mixing system return fluid temperatures. A boiler minimum temperature is set, and if the boiler sensor temperature is cooler than this setting while the boiler is firing, the control reduces the output from the mixing device.
Idling of the snowmelting system is needed in various situations. When the snowmelting system starts from a cold temperature, the time required for the system to reach the melting temperature may be excessive. Idling decreases this start-up time by maintaining the system at a lower than melting mode temperature. This feature is also useful to prevent frost and light ice formation. One should note that the operating cost changes when the system is in idling mode.
ConclusionProviding safety and reduced insurance costs and claims through snowmelting specification allows engineers to add value to their services. There are many different ways of piping and controlling a snowmelting system; however, by identifying the main components and their purposes, a designer or engineer can greatly simplify the piping and control design. The diagrams shown in this article have been applied many times in the field, with slight variations depending on application. Most well established manufacturers provide engineering tech support to designers of snowmelting systems. Viega also provides snowmelting classes covering design and installation practices.
Capitalizing on the increasingly widespread acceptance of snowmelting for both commercial and residential installations makes good business sense, as the safety and maintenance advantages to a building owner are numerous.