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Bosch Security Systems | 2011-02
Praesideo 3.5 | Installation and User Instructions | 7 | System Hardware Installation en | 306
P = power (W)
U = DC voltage (V)
I = DC current (A)
Because all the Praesideo units have built-in switched
mode power supplies, the power consumption is
virtually constant and within a certain range
independent of the supply voltage V. The lower V is,
the higher I will be.
33.5 Cooling capacity
A simple formula is used to calculate the required
cooling capacity of an air conditioner for a technical
room where Praesideo equipment is installed. It is based
on the fact that only power amplifiers and basic
amplifiers contribute to the heat generated in a room;
the contribution of other Praesideo equipment is
negligible. The maximum ambient temperature for
Praesideo rack-mounted equipment is 55 ºC.
Energy is power multiplied by time. The unit of energy
is Joule; a Joule is the same as a Watt*second.
Therefore, one Watt of dissipation inside the rack
during a period of one second results in one Joule of
energy.
The calculation is based on the LBB4428 amplifier,
since all other amplifiers are slightly more efficient. An
LBB4428 consumes 800 W during full power operation,
so with an output of 480 W (8 channels of 60 W), the
amplifier dissipates the difference (320 W). The
theoretical energy that is produced as heat inside the
unit is 320 Watt * 3600 seconds = 1,152 kJoule each
hour, or 27,648 kJ each day (24 hours). In reality,
however, the energy will be a lot less. The Praesideo
amplifiers are only allowed to be driven:
• at full power for 1 minute,
• at half power (-3dB) for 30 minutes for emergency
tones (these are generated at -3 dB level),
• continuously at quarter power (-6dB) for calls or
emergency announcements, or for BGM.
The continuous quarter power limitation gives enough
margin to use the amplifier well into the limiter, because
the crest factor of speech is around 8..10 dB, and for
music also at least 6 dB. So, this is a safe worst-case
margin.
Based on this, the maximum heat energy generated in a
day (24 hrs) is:
(800 W - 480 W) * 1 minute + (397 W - 240 W) * 30
minutes + (228 W - 120 W) * 1400 minutes = 320 W *
60 s + 157 W * 1800 s + 108 W * 84,000 s = 9,373,800
Ws = 9,374 kJ.
This is equivalent to a light bulb of 110 W for a single
Praesideo amplifier of any type. The power dissipation
of the amplifiers should be summed. To calculate in
BTU (British Thermal Units), divide the amount of kJ
by 1.055. To calculate in kcal, divide the amount of kJ
by 4.184. To calculate the heat production per hour
instead of per day, divide by 24.
So 9,400 kJ/day = 9.4 MJ/day = 2,240 kcal/day = 8,900
BTU/day = 390 kJ/hr = 93 kcal/hr.