heating the rooms created dissimilar airflow regimes within the different rooms.
The method of airflow measurement is different for both cases. Shaw used hot wire anemometers to measure the velocity of the air through the doorway, from which the airflow was calculated. The errors associated with the use of this type of instrument on the calculation of airflow rate were not stated.
The orders of magnitude of door openings are different for both cases. Shaws smallest door opening was fifty times larger than the smallest door opening used in the Project It may be that under these circumstances, the airflow regimes through the openings may not be of the same type,
and direct comparison cannot be made.
As stated earlier in this section, the location of the test rooms within the general layout of the hospital is not known. It may be that these rooms were influenced in some way by the effects of the weather, thus affecting the measured airflow rates.
No mention is made of temperature variation with time by Shaw. As no specialist heating system was mentioned which could prevent this, it must be assumed that none existed.
Temperature variation with time, especially at higher
temperature differences, is inevitable. How this was
included in Shaw's representative temperature difference is not stated.
6.6 Comparison of Theoretical and Empirical Temperature
Driven Airflows Through Doorways
This section compares the theoretical and empirically derived flow equations from sections 6.2 and 6.4, and are
summarised in Table 6.8
FLOW EQUATIONS nT3/h Door
pos 'n Simple theory Simple modified Empirical
Pos 1 Q=3.2 4 T A0.5 Q=3.5 AT"0.5 Q=1 A T A0.5 + 6
Pos 2 Q=4.9 4 T A0.5 Q=5.4 ^T"0.5 Q=4 A T A0.5 + 11
Pos 3 Q=15.1 AT~0.5 Q=19.6 4T~0.5 Q=19 AT~0.5 + 10
Table 6.8 Summary of Flow equations
The first noticable difference between the theoretical and empirical equations is the inclusion of a constant term, independent of temperature, in the empirical equations. This could be a measure of the amount of background air turbulence ( section 5.2.1 ), ever present, even when there is no net temperature difference between rooms. The use of radiators has been shown to be a source of turbulent flow within rooms (33) . If this is the case,
then the amount of turbulent flow measured is between 6
m A3/h to 11 irT3/h.
Perera (46) has shown that the equations as described by Lidwell indicate a turbulent flow of 43 0 nT3/h, eachway through a doorway of dimensions 1.9 m x 0.8 m ( height x width ) . The indications are therefore, that the amount of turbulent flow also depends on the door opening. This is only partly borne out by the empirical equations. Positions 2 and 3 indicate larger turbulent flows than Position 1, which is to be expected, but Position 2 shows greater flow than Position 3, even though 3 is larger than 2. By converting the empirical turbulent flows to the flow per unit area, these are 352, 423 and 125 nT3/h for Positions 1, 2 and 3 respectively. Conversion of Perera's figure new indicate specific turbulent flows of 282 nT3/h. In arriving at this figure, the assumption made by Perera is that of a constant turbulent air velocity through the doorway. This can only be assumed to be an approximate value however, since there are so many possible
permutations of conditions which can exist to create these turbulent velocities ( the thermal properties of walls, floors and ceilings of the enclosing rooms, dimensions and geometry of the rooms, method of heating of the rooms ) .
The temperature dependent terms for both the simple and simple modified theory show poor correlation at the smallest of the door openings. This may be because of the incorrect assumption of the door gap flow characteristics, such as the value of coefficient of discharge.
The correlation between the temperature dependent terms of both the empirical and theoretical equations appears to be good for Position 2. This agreement also appears to be good for the simple modified equations for Position 3. The agreement at this position for the simple equation is less good, showing an approximately 25% lower figure.
A contributory factor to the poor agreement between the theoretical and empirical equations, may be errors in determining the area terms within the theoretical equations, which are unknown. It is probable that because the method of measurement was the same for all three door positions, the errors in measurement will be proportionately greater at the smaller door openings than at the larger ones.
Conclusions of Chapter 6
It is possible to derive empirical equations for convective airflows through doorways. The correlation between these and simple theories of convective flow is
good for Positions 2 and 3. For the closed door, the
correlation is poor. The simple modified analysis gives better agreement with experimental results, especially at larger door openings.
Of concern is the lack of knowledge of the behaviour of the coefficient of discharge at different temperature differences, especially for the closed door position. The airflow measurements for this position would indicate better agreement with the theoretical equations had a coefficient of discharge equal to 0.19 had been used. A better knowledge of the coefficient of discharge could enable more accurate theoretical solutions to be determined.
The omission of turbulent airflows is a criticism of both simple theories. At lower temperature differences this appears to be a significant contributory factor to the total flow through doorways.
CHAPTER 7 SITE MEASUREMENTS OF TEMPERATURE DRIVEN