Melting snow and ice by heating pavements

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Melting snow and ice by heating pavements

MELTING SNOW AND ICE

BY

HEATING PAVEMENTS 1 CANADA - -- - - - - Ser

m

1

P.

A. Schaerer' B92 no.

55

c . 2

m e

January I 9 6 6 ,

KNALYZED

I ": _.

.

c: -; L;{, ;!

-

I D l V i S I a H O F B U I L D I N G R E S E A R C H N A T I O N A L R E S E A R C H C Q U N C I L O T T A W A

.

C A N A D A

1

I

MELTING SNOW AND ZGE

B Y

HEATING PAVEMENTS

P . A . Schaerer

Snow and ice removal by heating the pavement is a costly operation. Experience has shown that the capital cost of installations ranges from $1.00 to $ 6 . 0 0 per sq f t and operating c o s t s from 1 5 to

30 cents per sq f t per winter, or $10.00 to $15.00 per ton of snow.

The cost of plowing and haaling snow

in

major cities in Canada is

now between $0.50 t o $1.00 per ton. Removing snow and i c e by heating the pavement i s still considerably m o r e expensive than conventional methods.

It

is justified only at locations where other methods of snow removal would be difficult o r expensive, where high traffic safety must be maintained, or where o w n e r s of property are

willing to pay the high c o s t in order to eliminate manual snow removal.

Rising c o s t of labour and equipment, the lack of space for the s t o r a g e

of plowed snow

in

_{l a r g e cities, and the increasing need f o r better and}

more rapid snow and ice removal have contributed to the growing u s e of melting systems. The number of snow-melting systems installed

during the past years, and frequent inquiries received by the National

Research Council concerning them, indicated a need for information

for the design and c o s t benefit studies of such systems.

In

_{response to this need, this note h a s been prepared to:}

a) review and summarize the information that is available

b ) indicate where information can be obtained

c ) point out where knowledge is still lacking and research is required.

It

will consider only heating systems using buried pipes or e l e c t r i c

cables. Heat f a r melting snow may be supplied also by i n f r a r e d lamps. T h e r e is little technical information available, however, concerning the application of i n f r a r e d heaters for snow melting, which indicates a need for research on this method.

HEAT

REQUIREMENTS

FOR

MELTING SYSTEMS

The heat requirements f o r melting systems a r e discussed in

the ASHRdE Guide and Data Book (1) by Jorgensen (2) and Coulter and

I-Ierrnan (31.

Sensible heat, qs

This heat i s given by

s = rate of snow melting in in. of water per

hr

t = snow temperatare usually assumed equal to air

a

temperature, deg

F

This amount of heat is small.

Heat of fusion, q

m

qm = 746 x s ~ t u / ( sq f t ) ( h r )

This is the largest component: of the heat that must be supplied.

Heat of evaporation, q

e

Evaporation will occur at the road surface, particularly if

the snow is melted as it falls and the pavement is wet. Chapman (4},

using information from various publications on evaporation f r o m large

water surface s

,

derives the fallowing formula:

qe = 1075 (0.0201

V

+

0.055)(0.185

-

p av ) E5tu/(sq f t ) (hr)

v = w i n d velocity in mph

= v a p u r pressure of the air, in. of mercury.

'av

The accuracy of this formula has not been established for road surfaces,

but it appears to give reasonable values f o r the purpose of _{design of}

melting systems.

Heat 10s s by convection and radiation from b a r e pavements, qf

T h e r e i s a heat transfer from the road surface to the atmosphere

due to radiation, convection and conduction. This heat t r a n s f e r depends

on weather conditions and i s difficult to assess. Far purposes of d e s i g n , it may not be necessary to separate the influences of radiation, con-

vection and conduction. Chapman (4, 5 , 6 ) developed equations with

empirically determined coefficients far estimating the combined

contribution of these f a c t o r s . He recommends for wet surfaces:

%w = (0.23

Y

t 0 . 63)(tf

-

ta) 13tu/(sq ft)(hr) V = wind velocity, mph

t

f = temperature of road surface, deg F,

(usually taken as 33OF)

Hasselriis and Geiringer (7) found f r o m observations on a

Heat dissipated by convection and radiation from a d r y surface must

be supplied i f the melting system operates on a stand -by basis.

Chapman and Katunich ( 6 ) determined by measurements on a t e s t

s l a b with a d r y surface that

-

This formula appears t o give values that are too high. Considering the many factors that influence radiation and convection (slab temper-

ature, air temperature, type and colour of pavement, clouds, wind),

it would be expected that calculated values for heat l o s s will cover a

wide range, T h e r e appears to be a need f o r a study of heat loss by

convection and radiation from wet and dry slab surfaces in order t o

establish the equations required f o r design purposes.

Heat loss

by

evaporation, convection and radiation from snow surfaces,

9,

The t o t a l heat loss through evaporation, convection and radiation f r o m the snow surface cannot be higher than t h e _{heat transferred from}

the pavement through the snow cover. Using the conductivity of new

snow as given by Dossey ( 8 ) it follows that -

H

= snow depth, in.

tf

-

t

a deg F

H = temperature gradient, in.

This heat lass is usually small and _{can be neglected.}

Heat loss t o the ground and at the edges, q

a

A considerable amount of heat is l o s t from the pavement into

the soil underneath, f r o m the edges into the adjacent soil, or from the

underside of a bridge to the air. These losses depend upon the con- struction of the slab and temperature of the heating system. H a s s e l r i i s

and Geiringer ( 7 ) observed these l o s s e s t o be between 10 to 2 0 per cent

of the loss at the upper surface. Adlam

( 9 )

observed that about the same amount of heat was last into the ground as was required to heat the surface of a dry slab. The ASHRAE Guide and Data Book [ I )

upper surface. Jorgensen (2) presents a formula for calculating

the heat loss at the underside of a

slab,

As there appears to be no reliable information available that could

be

used as

a

basis f o r

estimating the downward and edge heat loss, and as t h i s loss is a

major factor in the design of melting systems, studies should be undertaken to

obtain the

information required t o calculate it more

accurately. Consideration should be given as well to studies on the

use of insulation to reduce this l o s s . Total heat requirement

The t o t a l heat that must be supplied far the melting of the snow depends on whether the snow is melted s o rapidly that the

pavement is kept clear at all times or snow is allowed to accumulate,

Chapman (5) has introduced the t e r m "free area ratio", wXch is used

also in the ASERAE

Guide

and Data

Book.

Area free of snow, s q ft Free area ratio, A =

r Total area, sq ft

A is 3 for snow-free pavements, and 0 for pavements that

x

are completely covered with snow.

Taking all sources af heat loss into consideration, the general equation

for the t o t a l heat output is:

Example

Heat requirements depend on

a

combination of climatic elements.

Following is a numerical example for average conditions:

Snawf all: 0.8 in.

/hr

= 0.08 in. waterlhr

Wind: 10 mph Temperature: 20 OF

A snow melting system would have to supply:

a) Heat t o raise temperature of snow, q

s

b) Heat of fusion,

gm

c} Heat a s s o c i a t e d with evaporation from the bare

pavement,

q

e

d) Heat loss due to convection and radiation, from

bare pavement, wet,

%w

~ t u / ( s ~ f t ) (hr)

2.5

from bare pavement, dry, qfd

from snow surface, 1 in. snow depth, q t e ) Assumed heat l o s s to ground ( 3 0 per cent of

heat loss at upper surface i f bare pavement

is maintained)

'

qg

The total required heat output would be:

a)

If

a snow cover is maintained on the pavement,

A = 0 and q = 88 I3tu/(sq ft)

(hr)

r 0

b) If a b a r e pavement is maintained,

A r = 1 and q o = 1 5 5 ~ t u / { s c ~ f t ) (hr).

CLIMATE INFORMATION

The calculation of the design heat output requires the

simultaneous consideration of four weather factors: (a] rate of

snowfall, (b) wind speed, {c) air temperature, and

(dl

r e l a t i v e

humidity. Chapman has made a _{frequency analysis for 3 3 cities in}

the U. S . A. of the required heat output f o r all occurrences of snowfall

for a period of several years (10).

He

used for

his

analysis records

of hourly snowfall, which are not available in Canada. The results

have been published in the ASHRAE Guide and Data Book (1).

Chapman's method appears to require considerable time and

labour even when computers are used. A detailed caLculation of the

heat output using accurate climatological data, however, appears to

be of little value when radiation and convection l o s s e s are determined

by simplified equations and the heat l o s s to the ground is estimated.

A simpler method of taking into account climatological data

would probably produce results with the same accuracy as obtained

with Chapman's analysis, F o r example, it might be just as good to

u s e a q'design snowstormc' to determine the design heat output. A

simple method should be developed f o r calculating the design heat

output for given climate conditions.

STANDARDS

FOR

SNOW

AND

ICE REMOVAL

- - - - -

The ASHRAE recommendations classify snow-melting installations as to the urgency f o r melting:

G l a s s

I

(minimum):

R e

sidential walks or driveways and

interplant areaways.

Clas s U- (moderate): Commercial (stores and offices)

sidewalks and driveways.

G l a s s LU {maximum): Toll plazas of highways and bridges, and

aprons and loading areas of airports. The classification depends upon the allowable rate of snow

melting. The design heat output f o r each C l a s s is determined from a

frequency distribution of snowfall rate s and the associated heat

necessary to keep the pavement bare

of

snow. The design output in

Glass 1 would give a bare pavement for 98 per cent of all hours with snowfall. The design output for C l a s s

III

_{prevents snow from}

accumulating on the pavement at practically all times. The differences

between C l a s s

1

and Class

Ill

are considerable. At some locations the maximum heat for C l a s s

IZI

_{is as m u c h as four times the maximum heat}

for C l a s s

I,

Reports of snow-melting systems that have been built at St.

Catharine s

,

Qntario ( 1 13, Saunde r s

St.

Lawrence Generating Station

3

121, and Calumet Expressway, Chicago (131, indicate that the design output

b a s e d on recommendations in the

ASHRAE

Guide and Data Book resulted

in unreasonably high capital c o s t . F o r these three installations, the

maximum heat output was reduced to half or even one -third of the o r i g i n a l estimate, giving a snow-free surface f o r about 90 per cent of the hours with snowfall. Apparently no disadvantages re sulted from this

modified d e sign, Experience with the operation of snow-melting systems in England (14) has indicated that traffic removes snow and slush when only

60

per cent of the snow has been melted. Experience suggests that the

ASHRAE

recommendations are too conservative

- -

except perhaps for sidewalks in cities and other small. but important areas

- -

producing

a snow-free pavement in a shorter time than is necessary.

It

may be

possible t o allow the snow t o accumulate to a depth of about 2 in. on

highways, city streets, parking lots and loading r a m p s without seriously

disrupting traffic. The accumulated snow would greatly reduce heat

10s s due t o evaporation, radiation and convection, and traffic may a s s i s t

in removal of slush. Information is required, however, on the amount

of slush that can be tolerated for given traffic conditions.

CHAUGTESUSTICS

OF

SNOW MELTING SYSTEMS

The c o s t of melting snow and i c e by heating the pavement depends on the amount of time the system will be required to be in operation.

system would be in operation at all hours when the air temperature is 3 2 ° F and below. The system would idle at a lower load than required for snow melting when no snowfall is occurring. Main- taining a pavement temperature of 32°F or mare would prevent ice

due to f r e e z i n g rain, freezing fog

and

traces of snow.

Systems designed by the British Road Research L a b o r a t o r y

(14) are switched on when the road surface temperature falls below

+3'C and there is moisture present on the road surface at the same

time. Other melting systems begin to operate only when snowfalls occur that a r e l a r g e enaugh to warrant snow removal. A system

idling whenever the temperature is below 32'F would have to operate almost continuously in cold regions, The high heat l o s s due to

convection and radiation from dry pavements, together with the long

idling t i m e would require an annual heat output which is up to 2 0 times

the annual heat output required f o r snow melting alone. F o r this

reason, systems in cold regions, such as most parts of Canada,

would normally be designed for intermittent operation, which m e a n s

they

would be in o p e ~ a t i o n only when snowfaU or other conditions causing i c e on road surfaces actually occur.

F o r systems that are operated intermittently, i d l i n g occurs during warm-up,for s o m e time after the snawfalL has ended,and when the system is started because a snowfall was expected but did not occur. The total idling hours f o r intermittent operation were estimated by

Chapman to be about twice the number of hours with snowfall. T h i s f i g u r e is probably low, Operating data of snow-melting systems at Worcester Airport, Massachusetts (1 5) and on the T o r o n t o Expressway

( 1 6 ) show that the idling t i m e is probably three to four times the hours of snowfall.

P a v e m e n t desisn requires pipes or cables t o be at least 2 in,

below surface for roads. For intermittent operation such pavements

require 8 to 1 2 hr time to reach a temperature of 3 2 ° F at the surface

when the weather is cold (0°F to

10mF)

and when the heat input necessary

for melting snow is available.

T o

have a s h o r t e r warm-up time would

require a higher heat output per hour and this might g o v e r n the design of the system and have a major influence on capital cost. A system with a low heat output for warm-up would require a warning long in

advance of snowfalls and other conditions that might result in

hazardous road surfaces. The ASHRAE Guide and Data Book d o e s not give sufficient information for design of ktermittently-operated systems.

D a t a upon which to b a s e calculations of warm-up time and the heat necessary for w a r m - u p are required.

Pavements are h e a t e d either by buried pipes containing a

circulating heating fluid or by buried electrical cables. _{Buried pipes} are used widely in the U . S . A . where heat is often supplied from large boiler plants associated with the _{heating of buildings. Systems with}

buried cables are probably m o r e suitable

in

_{Canada because t h e price}

of electrical power i s lower, and electrical systems a r e m o r e suitable

for intermittent operation. Data f o r the design of buried pipe s ysterns are available in references (11, (23, ( 3 ) and ( 9 ) . Information on

electrical systems that have been built is available in references ( z ) , (31, (l2), (14) and (I$), but there is no general guide or handbook f o r

the design of such systems.

Serious damage due to thermal s t r e s s e s h a s been observed

after several years of operation of melting systems, ( Z , 3 ) . Examination

of systems after several years of operation, particularly those that

have failed, is needed in order to discover weaknesses in the design and

causes of failures.

There is a need Ear defining the weather conditions that result

in icy road surfaces and require heating systems t o operate. Such

conditions must be forecasted up to 12 hr before they occur, depending

on the system, in order to allow adequate warm-up of the heating system.

COST

The capital and operating costs depend on local conditions.

Costs given in publications usually are not defined adequately: often it

is not shown whether given c a s t s are for installation only or include as

well control equipment, b o i l e r s , additional c o s t for paving, e t c . The capital c o s t s given range from $1.00 to

$ 6 . OQ

per sq f t of heated

pavement.

The operational costs depend on the climate, the degree to which the pavement is kept snow free and the time during which the heating system idles. U s i n g d a t a given by ASHRAE ( 1 ) , the heat

requirements for cities in the northeast isnd north central United States

were calculated to be between 1 5 and 36 kwh per sq f t per winter when

the system is contralled carefully (idling time, three times the number

of hours with snowfall). These conditions would correspond to conditions

in southern Quebec, eastern and southern Ontario. At a price of 1 cent

p e r kwh, the operating c o s t would range between $0.15 and $0.30 p e r s q f t per winter, which means it c a s t s between $10.00 and $15.00 to melt

one ton of anow. As i t may be difficult in many places to control the

the calculated values. The operating cost of the heating system in

the Toronto Expressway was $0.32 per sq f t in the winter of 1963-64

(1 6 ) .

RESEARCH NEEDS

The study of the design data for snow melting systems and the

published reports on the performance of such systems have indicated the need for further research in _{the following fields:}

I . Snow and Ice

(a) T o determine how much snow can be allowed to accumulate,

before traffic is hindered, in relation to weather and pavement

temperature on the road, on city streets, at inter sections, at parking lots, a n sidewalks.

(b)

T o

define the weather conditions that can result in ice on roads.

(c) T o establish, with ~ e f e r e n c e to given standards of snow removal, when a heating system must operate,

2. Meteorology

(a) Measurement of the heat loss by evaporation, radiation, and

convection for different conditions of the road and weather.

(b)

T o

develop a simple method lor calculating the design heat output for given climate conditions.

(c)

T o

supply the climatic data for the design of heating systems in Canada.

(d) T o design and develop the meteorological instruments required

for the control of melting systems.

3. SoifsandBavement

(a) T o determine heat losses into the ground both by mathematical

analysis and by observations

during continuous operation, during warm-up,

(b) T o obtain the necessary design information for the pavement and

insulation.

( c ) T o study pavements with heating systems in o r d e r to obtain

4, Mechanical and Ere c t r i c a l Engineering

(a} T o determine the most efficient position and spacing of pipes or cables in the pavement and e stablish technique s for making

the installation.

(b)

To

determine the characteristics

of

circulating fluid and electrical systems that w i l l , for given d e s i g n s , optimize:

capital cost,

operating coat,

warm-up time,

lifetime.

SUMMARY

The heat requirements f o r snow removal by heated pavements

depend on the following influences: rate sf snowfall,

air temperature, wind speed,

relative: humidity af air (influence is small), heat transfer from the pavement into the ground,

effect desired (bare pavement or snow cover allowed),

warm -up time,

idling time,

area of pavement,

frequency of snowfalls and icing.

W i t h these data available, the design output in I3tu/(sq ft)(hr) and the

total heat required during one winter c a n be determined.

W i t h the c o s t s of the energy known the economics of a

heating system may be assessed. It should be kept in mind that the high c o s t of snow and i c e control

by

heated pavements

is

due mainly to the heat loss from snow-free surfaces into the atmosphere and

from the pavement into the ground, particularly during the period

when the pavement is maintained at 326F prior to a snowfall,

REFERENCES

The fallowing references are a selection of the many publications

concerning snow and ice contra1 by heated pavements, and a r e the r e f e r e n c e s which give information that is useful for the design of such systems.

ASHRAE guide and data book,

1964.

Applications, Chapter 8 3:

Snow melting. American Society of Heating, Refrigerating and

Air-conditioning Engineers, hc.,

New

York.

Jorgensen, Roy and Associates. NOPL- chemical methods of snow and ice control on highway structures. National G o -3perative Highway Research P r o g r a m , Report

4,

Highway

R e

search Board,

Washington, D.C., 1964,

Coulter,

R.

G.

and 5, Herman. Control of snow and i c e by

inducted melting. The City College Research Foundation, New

Y o r k , 1964.

Chapman, W. P . Design of snow melting systems. Heating and

Ventilating, Vol. 49,

No,

4, April 1952, p. 95-102.

Chapman, W. P. Design conditions f o r snow melting. Heating

and Ventilating, Vol. 49,

No.

11, November 1952, p. 88-91.

Chapman, W . P . and S . Katunich. Heat requirements of snow melting systems. Transactions American Society of Heating and

Air-Conditioning Engineers, Vol. 62, 1956, p. 359-372.

Has selriis,

F.

and P .

L.

Geiringer. A i r c r a f t runway anow melting t e s t s using high temperature liquids. W r i g h t Air Development Center,

WADC

Technical Report 58- k 7 5 , Armed

Forces Technical information Agency, Arlington,

Va.

,

M a r c h 1958.

Dorsey,

N.

E. Properties of ordinary water substances in all its

phases. Reinholdpublishing Corporation, N e w York, 1940.

Adlam, T.N. Snow melting. The Industrial Press, New Y a r k ,

1 9 5 0 .

Chapman,

W

.

P. Calculating the heat requirements of a snow

melting s ystern. Air Conditioning, Heating and Ventilating,

Vol, 53, September 1956, p. 6 7 - 7 1 ; Vol. 53, October 1956, p.

81-84; Vol. 53, November 1956, p,

96-99;

Vol. 53, D e c e m b e r

1956, 71-74; Vol. 54, January 1957, p, 6 3 - 6 6 ; Vol. 54,

February

1957,

p. 101 -105; Vol. 54, March 1957, p. 5 4 - 7 7 ; V a l . 54, April 1957, p. 87-91; Vol. 54, May 1957, p. 72-76.

Ross, L. Snow melting system keeps Skyway's toll plaza clear. Heating, Piping and Air "Conditioning, March 1964, p. 99

-

107. Kobold, A. E, and G.

H,

West. Snow melting by electrical means.

13. Neal, G .

W,

N e w toll plaza snow melting systems keep

Skyway traffic moving. Heating, Piping and Air -Conditioning,

Vol. 30, September

1958,

p. 140-143.

14. W h i f f i n ,

A-

C . and

P.

J.

Williamson. Electrical

heating

of

roads

t o prevent the formation of ice and frost. Heating (London),

VOE.

2 5 (2061,

1963,

p. 41-47.

1 5. Turnbull,

F.

3 . Snow melting problems and how they w e r e solved.

ASHRAE

Journal, Vol. 3 , No. 1 , January 1961, p. 71-72.

16.

George, J.D. a n d C . S . W i f f e n . S n o w ~ d i c e r e m o v a l f r o r n r o a d surfaces by electrical heating. Preprint of paper presented at the 44th Annual Meeting of the Highway Research Board, January