Volume flow rate is characterized by the displacement of a given volume per unit of time. The international unit of volume flow is [m3/h].
In our professional sector not much happens in the space of one second. We will often indicate flow rates per hour, and sometimes per minute.
The common measuring units used in HVAC are:
– [m3/s], and [l/s] (litres per second)
– [m3/h], [l/h], [l/min]
The usual symbol for flow rate in general is: q
The usual symbol for volume flow is: qv
During the first phase it is not necessary to have the formulae to study this physical quantity.
Simply by using your common sense, resolve the following exercises:
Question
The average flow rate of a bath tap is 0.2 [l/s].
Convert this rate into [l/min], into [l/h], and into [m³/h].
12 [l/min]
720 [l/h]
0.72 [m³/h]
Explanations :
We know that there are 60 seconds in a minute.
Therefore the flow rate over 60 seconds (1 minute) is 60 times higher than that obtained in one second.
qv = 0.2 x 60 = 12 [l/min]
By applying the same reasoning we can obtain the flow rate expressed in [l/h].
1 hour = 60 [min]
The flow rate is therefore 60 times higher than that obtained in 1 [min]
12 x 60 = 720 [l/h]
To obtain the flow rate in [m³/h], it is simply necessary to convert litres to [m³].
1 [m³] = 1,000 [l] therefore : 720 [l] / 1,000 = 0.72 [m³]
Hence : 720 [l/h] = 0.72 [m³/h]
Observation :
You should always make sure that the units correspond, and undertake
all the conversions you deem necessary.
Question
A water tank of 200 [l] empties its hot water in 15 [min].
What is the flowrate of the drain tap, in [l/s]?
0.22 [l/s]
Explanation :
15 minutes = 15 x 60 = 900 [s]
We know that this water tank empties in 15 [min] (900 [s]) therefore the outflow rate is equal to :
200 [l] / 900 = 0.22 [l/s]
Question
A boiler manufacturer indicates a domestic hot water production capacity for his boiler of 12 [l/min].
This boiler feeds a storage tank of 300 [l].
How long does it take to fill the storage tank with hot water ?
300 / 12 = 25 [min]
Explanation :
15 minutes = 15 x 60 = 900 [s]
Every minute the storage tank fills with 12 litres; its capacity being 300 litres, it would need a production time of :
300 / 12 = 25 [min]
Question
A container of 0.75 [m3] is filled in 8 [s].
What is the filling rate in [l/min] ?
5.625 [l/min]
Explanation :
0.75 [m3] = 0.75 l x 60 = 5.625 [l/min]
(0.75/8) x 60 = 5.625 [l/min]
We know that every 8 seconds, we obtain the equivalent of 0.75 [litres], therefore in 1 second we will obtain 8 times less, and in 60 seconds (1 minute) sixty times more than the 1 second value.
Question
A leak from a toilet cistern is approx. 0.001 [l/s].
What quantity of water (in [m3]) would be lost each year if it is not repaired ?
What would be the annual cost of this leak based on a price of 4 [€] per [m3] of water.
31.5 [m3]
126 [€]
Explanation :
1 hour = 3,600 [s]
1 day = 3,600 [s] x 24 = 86,400 [s]
For 1 day the leak will be equal to 0.001 [l] x 86,400 = 86.4 [l]
For 1 year: 86.4 x 365 = 31,536 [l] or 31.5 [m3]
Cost of the leak over 1 year : 4 [€] x 31.5 = 126 [€]