Ventilation flow rate and external static pressure calculation

Objectives:

To calculate ventilation volumetric flowrate and ESP calculation for the ventilation duct

1. Ventilation volumetric flowrate calculation

First of all we write the general design criteria

General Design Criteria

A.1 Space reference (Toilet 2111)

A.2 Elevation (2047ft) 

A.3 Duct material type (Galvanized Iron Sheet)

A.4 Friction Loss (0.15 in.w.g)

Length = 3.4m

Width  = 2.3m

Height = 2.5m

No. of Air changes per hour = 7

Space volume (V) = L x W x H

V = 3.4 x 2.3 x 2.5

V = 19.55 m3

Now volume flow rate is given by below equation

v = (Space volume x 1000 x No of air changes per hr) / 3600

v = (19.55 x 1000 x 7) /3600

v = 38 l/s

If we want to convert it to CFM then multiply above figure with 2.119

v = 38  x 2.119

v = 81 CFM

2. ESP calculation for the ventilation duct

So we have calculated ventilation volumetric flowrate which is 81 CFM, now let's calculate external static pressure for ventilation duct

2.1 Length of straight duct for the farthest terminal

We have to get that length from the drawing. Let's say it's 7ft

2.2 Fittings equivalent feet of straight duct 

This we also need to take from drawing. Let's say we have fittings transition, Take-off and 90 deg elbow. Quantity of each fitting along with equivalent feet of straight duct is given below

Fitting type   Quantity   Equivalent ft of duct 

Transition              1                      5

Take-off                 1                      1

90 deg elbow        1                      10

So total fittings equivalent feet of straight duct is given below

2.2 FEFSD =5 + 1 +10 = 16ft

2.3 Total Length is sum of section 2.1 and 2.2

2.3 Total length = 7 +16 = 23ft

2.4 Duct Static Pressure is calculated below

Since we said in the beginning Friction loss considered for design as per section A.4 is 0.15 in.w.g and now we know the total length so duct static pressure is given below

2.4 SP =  0.15 x 23/100 = 0.034 in.w.g

2.5 Now we have to see additional friction losses due to accessories that's to be added in section 2.4 to get total static pressure

Accessories     Quantity       (in.wg)                            

Duct heater         0         0.13

Sound attenuator 0         0.50

Volume damper         1         0.08

2.5 So total accessories static pressure = 0.08

Hence the total Total external static pressure is given below 

ESP = section 2.4+ section 2.5

ESP = 0.034 + 0.08

ESP = 0.114 in.wg

For more details on ESP calculation please refer to video below. Let me know if you have any questions in the comment section below. Thanks