On Selecting the Operating Mode of Auxiliary Ventilation Fans

B. M. Titov

(Presented by Prof., Doctor of Technical Sciences I. A. Badashev)

The operating mode (capacity, head, and efficiency) of auxiliary ventilation fans is determined by the point of intersection of the network characteristic with the fan characteristic, plotted on the same scale. If the fan characteristic is given in terms of total head, then the characteristic of the ventilation duct is also plotted in terms of the drop in total pressure

\[ h=\left(pR+\frac{\rho}{2F^2}\right)Q^2, \tag{1} \]

where \(p\) is the leakage coefficient of the duct;
\(R\) is the resistance of the duct, \(\mathrm{kg\,sec^2/m^8}\);
\(\rho\) is the density of the ventilation air, \(\mathrm{kg\,sec^2/m^4}\);
\(F\) is the area of the outlet cross-section of the diffuser or of the delivery duct downstream of the fan, \(\mathrm{m^2}\);
\(Q\) is the fan capacity, \(\mathrm{m^3/sec}\).

The resistance of a blind working, in comparison with the resistance of the ventilation duct, is very small; therefore it is usually not taken into account. Nor should the dynamic head of the fan installation be neglected in formula (1), since this leads to significant errors, for example:

at an air velocity of \(10\quad 15\quad 20\quad 25\ \mathrm{m/sec}\),
the corresponding error is \(6\quad 14\quad 24\quad 38\ \mathrm{kg/m^2}\).

If the fan characteristic is given in terms of static head, then the duct characteristic must also be plotted in terms of the drop in static pressure

\[ h_{cm}=pRQ^2. \tag{2} \]

When several fans are connected to a common duct, their operating mode is determined from the combined characteristic, plotted with allowance for air leakage (intake) and the pressure drop in the duct between them.

The combined characteristic of fans connected in series must be plotted in terms of total head, since the combined sta-

the static pressure of the fans in this case is greater than the sum of their static pressures by the magnitude of the dynamic pressures of all the fans connected in series, except the last one, counted in the direction of the air flow [1, p. 186].

The connection scheme for fans joined in series, in order to reduce air leakage (suction), must be selected with allowance for the type of ventilation duct. For example, with self-sealing flexible ducts (made of rubberized fabric, plastics, etc.), the second and subsequent fans, in order to increase the mean pressure in the duct, should be installed side by side; whereas with rigid ducts (metal, wooden, etc.), in order to reduce the mean pressure (vacuum) in the duct, they should be placed at some distance from one another, but in such a way that no air recirculation occurs [2]. When ventilating very long workings, it may turn out that the internal pressure in a flexible duct reaches a value inadmissible under the conditions of the mechanical strength of the pipes. In this case, the additionally connected fans will have to be mounted at a distance from one another.

The resultant characteristic of fans operating in parallel may be constructed either by total or by static pressure, but in correspondence with the representation of the duct characteristic. Parallel connection of local-ventilation fans is encountered very rarely.

The resistance of the ventilation network when ventilating a blind working varies from zero to some maximum determined by the type, diameter, and length of the ventilation duct. The fan is selected according to the most difficult operating condition, which corresponds to ventilating the blind working at the maximum length of the ventilation duct. As the resistance of the duct changes, the operating condition of the fan changes; therefore the economy of a local-ventilation fan installation should be judged not by the maximum efficiency, but by the weighted-average efficiency.

With the suction method of ventilating a blind working, in order to increase the capacity and static efficiency, it is recommended that a diffuser or a branch pipe 2–3 duct diameters long be installed on the delivery side of the fan. Consequently, the noise suppressor of the Giproglemash design, installed behind the fan, is at the same time also a diffuser.

Let us consider the operation of a fan with a monotonically decreasing pressure characteristic on a duct of variable length. To make the example concrete (Fig. 1), let us take a local-ventilation fan of type VP—4 and a flexible duct of type M. The operating condition of this fan, determined graphically for various lengths of the ventilation duct, is given below:

Taking the minimum permissible fan capacity as equal to \(1.25\ \text{m}^3/\text{s}\), we find the dependence \(\eta = f(l)\), shown in Fig. 2.

The dependence of the fan efficiency on the length of the ventilation pipeline (Fig. 2) makes it possible to determine the time-weighted average efficiency of the fan installation

\[ \eta_{\mathrm{avg}}= \frac{\eta_1 t_1+\eta_2 t_2+\eta_3 t_3+\ldots+\eta_z t_z} {t_1+t_2+t_3+\ldots+t_z}. \tag{3} \]

Here \(\eta_1,\ \eta_2,\ \eta_3,\ldots,\eta_z\) are the average values of the fan efficiency, taken respectively over the time intervals \(t_1,\ t_2,\ t_3,\ldots,t_z\), during which the efficiency curve can, with sufficient accuracy for practical purposes, be replaced by a straight line.

In Fig. 1: fan total pressure, kg/m\(^2\); fan efficiency, %; fan capacity, m\(^3\)/s; \(H=f(Q)\); \(\eta=f(Q)\); \(\ell=0,\ 50,\ 100,\ 200,\ 300\).

Fig. 1. Determination of the operating modes of a VP-4 fan (at \(P_{\mathrm{in}}=4.0\) atm and \(Q_{\mathrm{air}}=4.6\ \text{m}^3/\text{min}\)) for a rubberized type-M pipeline of 400 mm diameter and lengths of 0, 50, 100, 200, and 300 m.

In Fig. 2: fan efficiency, %; length of ventilation pipeline, m; \(\eta=f(\ell)\); \(\eta_p=0.85\,\eta_{\max}\); \(\eta_{\max}\).

Fig. 2. Character of the variation in the efficiency of a VP-4 fan when ventilating a blind working of variable length.

For the particular case considered by us, at a rate of advancing the blind working of \(150\ \text{m/month}\), we have

\[ \eta_{\mathrm{avg}}= \frac{38\cdot10+60\cdot10+65\cdot10+64\cdot10+59\cdot10+55\cdot10} {10+10+10+10+10+10} =56\%. \]

This value of the time-weighted average efficiency has been obtained without taking into account regulation of the fan capacity; therefore, in actual-

under operating conditions the fan installation may be somewhat more economical.

From what has been set forth above (Figs. 1 and 2) it is clear that, in order to increase the weighted-average efficiency of an auxiliary-ventilation fan installation, the fan must operate at regimes corresponding to the ascending and descending branches of the efficiency curve. The normal calculated reduction of efficiency on the left (Fig. 1), ascending branch of the curve \(\eta=f(Q)\), by analogy with the determination of the industrial-use zone of main fans, may be limited by the value

\[ \eta_p = 0.85\eta_{\max}. \tag{4} \]

Thus, when selecting an auxiliary-ventilation fan, one should strive to ensure that its operating regime lies within the industrial-use zone on both the right and the left branches of the efficiency curve. To ensure a stable operating regime of the fan installation on the left, ascending branch of the efficiency curve, the fan must have a monotonically decreasing pressure characteristic.

LITERATURE

1. Ovchinnikov V. N. Parallel and series operation of fans. In: Collection of Articles on Industrial Aerodynamics and Fan Engineering, ed. K. A. Ushakov. Tula: TsAGI, No. 211, 1935.

2. Voropaev A. F. Operation of fans connected in series to a single pipeline. Mining Journal, No. 1, 1917.