Non- Air-Conditioned Room Ventilation

Non-Air-Conditioned Room Ventilation

A young engineer reached out to me to help verify if his provision for a pump room ventilation is adequate.

Here is what he has done.

Pump room dimensions
Width: 3.0m
Length: 5.0m
Height: 3.5m
Required air change: 10 per hour.

Ventilation rate: 3.0m x 5.0m x 3.5m x 10 air change per hour ~ 525 m³/hr ~ 145L/s.

If you put in 10 air change per hour, my question will be why ten air change per hour? Why not 11 or 9? Too many time I heard engineers and contractor sizing pump rooms, electrical switch rooms and substation or asked if the air change is adequate. My answer is always the same, ” I don’t know”. I do not know because “I don’t know”, but instead I do not have enough information to ascertain for sure the adequacy of the provision.
I will need answers to the following,

What is the internal heat generated?
What may be the maximum internal temperature?
The equipment inside the room dictates it; in other words, what is the safe temperature that will not cause failure.
What is the maximum ambient temperature?
What is the layout and configuration of the intake and exhaust?

Example

Say I have a 50kW diesel pump with an overall efficiency of 45% in the said room.

The required ventilation

First thing first, we will determine the minimum air required for combustion ~ 0.1 m3 of air/min/brake kW (2.5 ft3 of air/min/bhp).

So, the minimum combustion air required ~ 0.1 m3 of air/min/brake kW x 37kW ~ 4 m3/min ~ 240 m3/hr ~ 67L/s.

Next, the heat generated is approx 45% x 50kW ~ 22.5kW; some heat will be discharged as flue and mostly removed through the radiator.

The permissible room temperature rise depends on the ambient as well as the maximum room temperature that will not cause component failure. Most modern electronic boards have a max temperature rating between 50 to 55°C; let’s err on the safe side using 55°C.

And suppose design ambient temperature is 38°C, our permissible room temperature rise is 12K.

Ventilation rate required ~ 22.5kW/(1.099 kg/m3 x 0.017 kW.min/kg.K x 12K) + 5 m3/min ~ 105.5 m3/min ~ 6,300 m3/hr ~ 1,750 L/s. This will be approx 120 air change per hour.

Note: In many industrial ventilation designs, we use 5°C and a maximum ambient temperature of 44°C. The maximum permissible room temperature is 49°C. And under many circumstances, we air-condition these rooms.

On the other hand, if the diesel pump selected has an overall efficiency of 75%, heat rejected is approx 12.5kW.

then, the ventilation rate required ~ 12.5kW/(1.099 kg/m3 x 0.017 kW.min/kg.K x 12K) + 5 m3/min ~ 61 m3/min ~ 3,650 m3/hr ~ 1,000 L/s. This will be approx 70 air change per hour.

In other words using air change per hour to determine ventilation rate in pump rooms, utility rooms, the electrical room is nothing but a stab in the dark.

Let’s turn our attention to F in the formula above; F is the routing factor. F is about the location of the supply and exhaust air. The calculated ventilation rate will need to increase according to the images below.

Images source: cat.com/power-systems

In summary, we need to use the internal heat load to calculate the required ventilation rather than merely assigning an arbitrary air change per hour. Does it make you wonder why a lot of pump rooms and switch rooms are scorching during summer?

Non- Air-Conditioned Room Ventilation

Non-Air-Conditioned Room Ventilation

Example

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