Snow and Ice Melting Systems: Factors Influencing Design (Part II)

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Design of a snow and ice melting (SIM) system is a bit more complicated than design of a radiant heating system, since the SIM output loads are based on several factors, not just the outdoor design temperature. By “loads” we mean the amount of heat required to operate the system.

 

 

 

 

 

 

Some designers also call this the “heat flux” and it is usually expressed as “Btu per ft2 of area per hour” or “Watts per m2 per hour.” For example, 125 Btu/ft2 per hour is a SIM load required by some residential and commercial systems.

 

 

Actual SIM loads are based on melting snow into water and evaporating water into vapor, leaving outdoor surfaces dry  in some designs, the majority of the water is expected to drain off. Factors which affect SIM loads are:

 

– air temperature when snowing/melting

 

– rate of snow fall

 

– snow density

 

– wind velocity

 

– humidity level of the atmosphere

 

– slab temperature at start of snowfall

 

and even the “apparent sky temperature.”

 

 

SIM system in operation at a popular museum in Washington, DC.

SIM system in operation at a popular museum in Washington, DC. Click on this picture to read more…

 

Much of the information available to SIM designers comes from ASHRAE (American Society of Heating, Refrigeration and Air Conditioning Engineers). REHAU designers have access to this information.

 

A few years ago, ASHRAE dropped the Class I, II, III scheme for sizing loads and instead published tables  showing “Frequencies of snow-melting surface heat fluxes at steady state conditions” for major cities. The table goes from 75 to 100% frequency, and lists SIM loads for various Free Area Ratios.  Once the designer selects the most appropriate Free Area Ratio and Frequency Distribution, SIM loads can be selected from a published table. For cities not found in that table, a series of calculations will estimate the loads based on historical weather data for that location.

 

REHAU’s new Technical Bulletin 250 “SIM Load Selection Table” is provided to give suggested values for Free Area Ratio and Frequency Distribution for SIM designs across North America to meet typical customer expectations.

 

Once the SIM load is known for a project, the next design steps are:

 

– Estimate downward losses

 

– Calculate total loads for the heat source (typically a high-efficiency boiler)

 

– Design the PEX piping system for delivering the Btu requirement; this includes selecting pipe size, spacing and circuit lengths which are typically 3/4 in. PEX pipes at 8 in (20 cm) spacing with circuit lengths ranging from 200 to 350 ft (60 to 110 m)

 

– Select the type of heat transfer fluid (usually propylene glycol mixed with water)

 

– Calculate the flow rate for the SIM piping system to deliver the Btu requirement

 

– Determine the head loss through pipes, manifolds, heat source and other equipment

 

– Select the right circulator and other hydronic components, such as expansion tanks

 

 

From the designer’s perspective, once the SIM load is known, the rest of the steps are straightforward.

 

 

 

By Lance MacNevin