National Research Conseil national B*B Council Canada de recherches Canada 

Institute for I nstitut de 

Research in recherche en 

Construction construction 

Elevator Piston Effect and the Smoke 

Problem 

by John ti. Klote and George Tamura 

Reprinted from 

Fire Safety Journal 

Vol. 11, No. 2, 1986 

(IRC Paper No. 1485) 

p. 227 - 233 

Price $3.00 

NRCC 28445 

Canad3 

Fire Safety Journal, 11 (1986) 227 - 233 227 

Elevator Piston Effect and the Smoke Problem 

JOHN H. KLOTE 

Center for Fire Research, National Bureau of Standards, Gaithersburg, MD 20899 (U.S.A.) 

GEORGE TAMURA 

Division of Building Research, National Research Council of Canada, Ottawa, Ont. K1A OR6 (Canada) 

(Received May 1,1986; in final form June 13,1986) 

SUMMARY 

The material in this paper is part of a joint 

project between the United States and Canada 

to evaluate the feasibility of elevator evacuation of the handicapped. This paper presents 

an analysis for the pressure differences produced by elevator car motion and it presents 

flow coefficients for flow around cars in 

elevator shafts based on test data. The results 

of an experiment to verify this piston effect 

analysis are presented. Also, practical considerations concerning piston effect and 

elevator smoke control are presented. An 

equation is developed to determine the upper 

limit of the pressure difference across an 

elevator lobby caused by piston effect. 

INTRODUCTION 

In most elevator lobbies in North America, 

there are signs indicating that elevators should 

not be used in fire situations; rather that stairs 

should be used. Unfortunately, some people 

cannot use stairs because of physical handicaps. The use of elevators is a potential 

solution to these problems. Logistics of 

evacuation, reliability of electrical power, 

elevator door jamming, and fire and smoke 

protection are long-standing obstacles to the 

use of elevators for fire evacuation. All of 

these obstacles except smoke protection can 

be adressed by existing technology as discussed by Klote [ 11. 

the United States and the National Research 

Council of Canada (NRCC) are engaged in a 

joint project to develop smoke control 

technology for elevators. The initial report 

The National Bureau of Standards (NBS) in 

f 21 of this project contained discussions 

and evaluations of elevator smoke control 

systems. The transient pressures due to 

‘piston effect’ when an elevator car moves 

in a shaft is a concern of building designers 

relative to elevator smoke control. This paper 

addresses piston effect and evaluates it with 

respect to elevator smoke control. The term 

‘smoke control’ is used to mean the limiting 

of smoke movement by pressurization produced by mechanical fans. This meaning has 

attained some level of acceptance in North 

America. 

ANALYSIS OF PISTON EFFECT 

For short intervals when an elevator is 

traveling away from the fire floor, piston 

effect reduces the pressure difference from 

the elevator lobby to the rest of the building. 

In extreme cases this could result in smoke 

infiltration into the elevator lobby. 

The following analysis is intended to 

provide an understanding of piston effect and 

its effect on the pressure difference from the 

elevator lobby to the building. Also, the 

analysis may be useful in evaluating smoke 

movement in buildings when elevators are 

used for fire fighting and for rescue. For the 

sake of simplicity, buoyancy, wind, stack 

effect, and the HVAC system have been 

omitted from this analysis. Omitting stack 

effect is equivalent to stipulating that the 

building air temperature is equal to the outside air temperature. 

A downward-moving elevator car forces 

air out of the section of shaft below the car 

and into the section of shaft above the car as 

illustrated in Fig. 1. The law of conservation 

Elsevier Sequoia/Printed in The Netherlands 
228 

of mass can be written for the volume, Qa, 

above the car 

i Net mass flow Rate of mass change 

(into a volume, Qa) = ( within volume, Qa 

where 

moa = mass flow rate from outside to volume, 

mha = mass flow rate from below the elevator 

p 

Qa 

Car, Qb, to volume, Qa 

= air density within shaft. 

The volumes Qa and Qb are changing as the 

elevator car descends. For a downward 

elevator car velocity, V, and a cross-sectional 

area of the shaft, A,, the derivative of the 

volume, &a, can be expressed as 

- =A,V dQa 

dt 

The air density is essentially constant 

within the shaft. Therefore, substituting eqn. 

(2) into eqn. (1) yields 

Modern elevators operate at various speeds 

up to a maximum of ZOO0 fpm (IO ms-l). 

Strakosch [ 31 discusses elevator velocity and 

acceleration. In most cases an elevator will 

accelerate such that within a distance of one 

floor the car will be near or at its rated speed. 

Because acceleration time is relatively short 

and its influence on piston effect is minor if 

any at all, acceleration is eliminated from this 

analysis. 

To expedite the analysis, the flow areas are 

chosen such that they are the same for each 

floor of the building and that the only vertical 

airflow in the building is within the elevator 

shaft. The mass flow rate from the outside to 

Q, is 

where 

Na = number of floors above the car 

C = flow coefficient 

A, = effective flow area per floor between 

Po = outside air pressure 

Pa = shaft air pressure above the car. 

the shaft and the outside 

Pressure difference due to elevation (hydrostatic pressure) has been eliminated from 

eqn. (3) because there is no indoor-to-outdoor 

temperature difference. The effective flow 

area of a system of flow paths is the area that 

results in the same flow as the system when 

subjected to the same pressure difference. The 

ASHRAE smoke control manual [ 41 presents 

a detailed discussion of effective flow areas. 

For the space below the car, the mass flow 

rate from Qb to the outside is 

where 

Nb = number of floors below the car 

pb = shaft air pressure below the car. 

Because the absolute pressure in the shaft is 

nearly constant, the flow rate entering the 

shaft can be considered equal to the flow rate 

of mass leaving the shaft. Thus, equating the 

right-hand sides of eqns. (3) and (4), cancelling like terms and rearranging yields 

The pressure drop from the bottom to the 

top of a moving elevator car is accounted for 
229 

by disturbances in the flow caused by obstructions such as the car edges and protrusions from the car and by friction. Again, 

neglecting hydrostatic pressure difference, 

the mass flow rate from below the car to 

above it can be expressed as 

where 

Af = free flow area in the shaft around the car 

or cross-sectional area of the shaft less 

the cross-sectional area of the car 

C, = flow coefficient for flow around the car. 

To evaluate the flow coefficient, C,, tests 

were run on a twelve-story, two-car elevator 

shaft at the NBS administration building. A 

discussion of these tests is provided in the 

Appendix. For one car traveling in the twocar shaft the flow coefficient was 0.94, and 

for two cars traveling side-by-side together 

the flow coefficient was 0.83. The case with 

the two cars moving together was tested to 

obtain an approximation of a car moving in 

a single car shaft. 

Combining eqns. (la), (3), (5) and (6) 

yields 

(7) 

For the flow paths illustrated in Fig. 1, the 

effective flow area per floor of the three paths 

in series from the shaft to the outside is 

1r2 

1 A,= - + - + - 

[A: Ai A; 

where 

Ad = leakage area between the shaft and the 

Ali = leakage area between the elevator lobby 

elevator lobby 

and the building 

Ai, = leakage area between the building and 

For paths in series the pressure difference 

across one of the paths equals the pressure 

difference across the system times the square 

of the ratio of effective area of the system to 

the flow area of the path in question. Thus, 

for flows above the elevator car, the pressure 

difference, APE, between the lobby and the 

building is 

the outside. 

Substituting eqn. (7) into this yields 

This relation expresses the pressure difference, APb, produced above a downwardmoving car as a function of the car velocity, 

the building flow paths and the position of 

the car. 

PISTON EFFECT EXPERIMENT 

To test this calculation, experiments were 

conducted on an elevator in a hotel in 

Toronto, Ontario, Canada. The elevator shaft 

was 15 stories tall with a height between 

floors of 8.53 ft (2.60 m). The flow areas and 

car velocity are listed in Table 1. Flow areas 

A,,, Ab, and Ai, were determined by pressurization tests. The areas A, and Af were 

evaluated by the same method as was used in 

the Appendix. The car velocity was based on 

the heights between floors. The effects of 

wind, buoyancy, stack effect and the HVAC 

system were not significant during these tests. 

The techniques of the experiments are those 

that have been used for many pressurization 

tests conducted by the NRCC [ 5 - 81. 

TABLE 1 

Experimentally determined data for an elevator in a hotel in Toronto, Ontario, Canada 

A,1, area between shaft and lobby 

A,i, area between lobby and building 

Ai,, area between building and outside 

A,, cross-sectional area of shaft 

Af, free area of shaft 

V, velocity of elevator car 

1.41 ft2 

0.97 ft2 

4.85 ft2 

59.37 ft2 

20.96 ft2 

341 fpm 

0.131 m2 

0.0901 m2 

0.450 m2 

5.515 m2 

1.947 m2 

1.73 m/s 
2 30 

0 

I 

LI 

N 

.- 

v 

w 

W 

z 

w 

IY 

w 

LL 

L 

R 

w 

CK 

3 

m ul 

w 

(Y 

H 

a 

.06 

.04 

.02 

R - 

0 5 10 15 20 25 

TIME (sec) 

Fig. 2. Pressure difference, &, across the elevator lobby of a Toronto hotel due to piston effect. Conversion 

factor: 1 in H20 = 248.8 Pa. 

The measured pressure difference, APE, for 

the elevator car descending from the top floor 

is shown on Fig. 2. The pressure difference 

was measured at the floor level of the top 

floor, and the pressure peak occurs as the top 

of the car passes the differential pressure 

transducer. With the exception of some 

pulsations the AP, decreases as the car 

descends. Also Fig. 2 shows APE calculated 

from eqn. (10) using the measured data from 

Table 1. The calculations are in good agreement with the measurements. The measured 

pulsations are believed to be the result of 

effective change in shaft size as the car passes 

the housing for the automatic doors. 

MAXIMUM VALUE OF APli 

Examination of eqn. (10) shows that APli 

cannot exceed the limits of 

where the subscript u denotes the upper limit. 

This relation assumes that the elevator 

piston effect is the only driving force of air 

movement in the building and that vertical 

flow can only exist in the elevator shaft. This 

relation is for elevator shafts which are unvented or for which the vents are closed. 

Equation (11) was derived for a pressure 

difference APIi, on floors above the car for 

a descending car, however it is also valid for 

floors below an ascending car. Equation (11) 

can be used to estimate the magnitude of 

pressure difference across the elevator lobby 

due to piston effect. 

dependent upon V, A, and Af. For example, 

Fig. 3 shows the calculated relationship 

between (APE), and V due to one car moving 

in a single car shaft, a double car shaft and a 

quadruple car shaft based on the flow areas 

listed in Table 2. These flow areas are based 

on the measured values of A, from the tests in 

the NBS administration building reported in 

the Appendix and average values from Appendix C of the ASHRAE smoke control 

manual for a building with a floor size of 

46.0 ft X 222 ft (14.0 m X 67.7 m) and 10.17 

ft (3.099 m) between floors. Leakage values 

for walls and floors listed in the ASHRAE 

smoke control manual are based on measurements by the National Research Council of 

Canada [5 - 81. 

As expected the (AP,), is much greater in 

the case of the single shaft. It seems a safe 

recommeridation that single car shafts should 

not be used for fire evacuation except when 

car velocities are very low. Freight elevators 

are of concern because they are often in single 

car shafts. In the above example, the same 

shaft dimensions were used for all the car 

velocities for the purpose of comparison. 

The pressure difference, (APE), , is strongly 
231 

0- 

.25 

- 

0 

N 

= .20 

c - 

v 

W 

w .15 Z 

w 

fY 

W 

L 

a 

W 

fY 

3 

Ln 

w 

Ly 

11 

.18 

m .05 

// I I 

FOR SHAFT AND 

BUILDING FLOW 

AREAS SEE TABLE 2. 

SINGLE CAR 

CRR VELOCITY (fpm) 

Fig. 3. Example pressure differences from building to elevator lobby due to piston effect. Conversion factors: 1 in 

H20 = 248.8 Pa; 1 fpm = 0.00508 ms-'. 

TABLE 2 

Flow areas for elevator piston-effect analysis* 

ft2 m2 

Ad, area between shaft and lobby 

AIi, area between lobby and building 

Ai,, area between building and outside 

For single car shaft 

A,, cross-sectional area of shaft 

Af, free flow area around car 

1.60 

0.42 

0.54 

60.4 

19.4 

0.149 

0.0390 

0.0502 

5.61 

1.80 

I " I W"C.."I~ C.UI v,ru,c 

A,, cross-sectional area of shaft 120.8 11.22 

For quadruple car shaft 

A,, cross-sectional area of shaft 

Af, free flow area around car 

I ".V 

241.5 

200.5 

22.44 

18.63 ~~ - 

*For a single car shaft a value of C, = 0.83 was used. For double and quadruple car shafts a value of C, = 0.94 

was used. 

Minimum shaft dimensions for electric, 

hydraulic, hospital and observation elevators 

are listed by the National Elevator Industry, 

Incorporated [9]. An analysis of pistonef f ec t-induced pressure differences during 

Toronto hotel experiment. Piston effect is 

not a concern for slow-moving elevator cars 

in multiple car shafts. However, for fast cars 

in single car shafts, the piston effect can be 

considerable. Equation (11) can be used to 

evmuaze me magniwue 01 pressure uiiieieii~eb 

produced by piston effect for specific applications. 

a future paper. 

CONCLUSIONS LIST OF SYMBOLS 

The piston effect analysis was in very good A area 

agreement with the measurements from the C flow coefficient 
232 

m 

N 

P 

AP 

Q 

t 

V 

P 

mass flow rate 

number of floors 

pressure 

pressure difference 

volume 

time 

elevator car velocity 

density 

Subscripts 

a above elevator car 

b below elevator car 

0 outside 

S shaft 

e effective 

f free flow around car 

1 elevator lob by 

i building 

U upper limit 

REFERENCES 

J. H. Klote, Smoke control for elevators, 

J. H. Klote and G. Tamura, Smoke control and 

fire evacuation by elevators, ASHRAE Trans., 92 

(1986) Part 1A. 

G. R. Strakosch, Vertical Transportation: Elevators and Escalators, Wiley & Sons, NY, 2nd Edn., 

J. H. Klote and J. W. Fothergill, Design of Smoke 

Control Systems for Buildings, ASHRAE, 

Atlanta, GA, 1983. 

G. T. Tamura and C. Y. Shaw, Studies on exterior 

wall air tightness and air infiltration of tall buildings, ASHRAE Trans., 82 (1) (1976) 122 - 134. 

G. T. Tamura and A. G. Wilson, Pressure differences for a 9-story building as a result of chimney 

effect and ventilation system operation, ASHRAE 

Trans., 72 (1) (1966) 180 - 189. 

G. T. Tamura and C. Y. Shaw, Air leakage data 

for the design of elevator and stair shaft pressurization systems, ASHRAE Trans., 83 (2) 

G. T. Tamura and C. Y. Shaw, Experimental 

studies of mechanical venting for smoke control 

in tall office buildings, ASHRAE Trans., 86 (1) 

Vertical Transportation Standards - Standards 

for Elevators, Escalators and Dumbwaiters, 

National Elevator Industry, Inc., New York, NY, 

1983. 

ASHRAE J., 26 (4) (1984) 23 - 33. 

1983, pp. 122 - 127. 

(1976) 179 - 190. 

(1978) 54 - 71. 

performed on a twelve-story, two-car elevator 

shaft at the NBS administration building. The 

test concept was to eliminate leakage from 

the elevator shaft so that the flow across the 

elevator cab would be 

The mass flow pressure relation across the car 

is given by eqn. (6) 

%a = AfCc J2P(P, -Pa) (6) 

Combining eqns. (Al) and (6) and solving for 

the flow coefficients 

A,V cc = 

Af //- 

This relation is for a downward-moving car, 

however a similar relation can be developed 

for an upward-moving car. 

The elevator tests had the mechanical room 

located above the shaft with holes in the shaft 

for the cables and vents. Because the cable 

could not be sealed during tests, both the 

shaft and mechanical room were sealed by 

taping over all cracks and gaps around doors 

and blocking off all vents to the outside. The 

areas, A, and Af, were calculated according to 

the convention illustrated in Fig. AI. 

Because of difficulties in changing the car 

velocity, tests were limited to two car velocities, and data from these tests are listed in 

.O------i 

SHAFT 

WALLS 

LSTEEL BEAM HOUSING FOR 

AUTOMAT IC 

DOORS 

Fig. Al. Convention used for determination of A, 

and Af. A, = DG and Af = DG - NEF where N = 1 for 

a single car moving and N = 2 for both cars moving 

together. Note that the area, Af, does not include the 

small area between the car and the door housing 

because it is felt that the contribution of this area is 

insignificant. 

APPENDIX 

Evaluation of elevator car flow coefficient 

In order to determine the flow coefficient 

for flow around an elevator car, tests were 
233 

TABLE A1 

Data and results from car flow coefficient tests 

Shaft temperature 76 OF 24 "C 

Tests with 1 car moving 

AS 

Af 

Pb -Pa at V = 496 fpm (2.52 m/s) 

Pb -Pa at V = 330 fpm (1.68 m/s) 

cc 

Tests with 2 cars moving together 

4 

A f 

cc 

Pb -Pa at V = 496 fpm (2.52 m/s) 

Pb -Pa at V = 330 fpm (1.68 m/s) 

120.8 ft2 

79.8 ft2 

0.039 in H20 

0.016 in H20 

0.94 

120.8 ft2 

38.8 ft2 

0.21 in H20 

0.076 in HzO 

0.83 

11.22 m2 

7.41 m2 

9.8 Pa 

4.0 Pa 

0.94 

11.22 m2 

3.60 m2 

53 Pa 

19 Pa 

0.83 

Table Al. Tests were performed with both 

cars moving together. This was done in an 

attempt to simulate the effect of a single car 

moving in a single car shaft. 

The flow coefficients listed in Table A1 

were calculated from the greater car speed 

because the higher speeds are of greater 

concern in regard to piston effect. However, 

the flow coefficients calculated at the lower 

speed differ only by 4% for a single car 

moving and by 10% for two cars moving 

together. This tends to support the treatment 

in the paper of the flow coefficient as being 

constant with respect to velocity.