Ventilation

Question 1

What are the three basic methods of room ventilation?

Answer 1

Upwards supply (displacement ventilation), downwards supply, crosswise supply.

Question 2

What is the driving factor for ventilation volume requirements for human occupancy?

Answer 2

The removal of body odours.

Question 3

What are the reasons for ventilating a space

Answer 3

Introduction of oxygen, removal of carbon dioxide, removal of moisture, removal of contaminants such of VOCs.

Question 4

What are the results of poor diffusion?

Answer 4

Draughts, stagnation, poor air quality, large temperature gradients, noise.

Question 5

What are the three basic methods of room air diffusion?

Answer 5

Upwards supply (displacement ventilation), downwards supply, crosswise supply.

Question 6

In what locations in a room are extract grills recommended?

Answer 6

In a stagnant zone, close to an excessive heat source, close to a cold source such as a window, at a point of local low pressure such as the centre of a circular ceiling diffuser, discharging horizontally.

Question 7

In what locations should extraction grills not be placed.

Answer 7

Close to sources of conditioned air such as supply grills, radiators, fan coil units.

Question 8

Explain the coanda effect

Answer 8

If air is released towards a surface its will cling to that surface as it moves forward

Question 9

What is the advantages and disadvantages of displacement ventilation?

Answer 9

Advantages Because it directly supplies the occupies area without mixing with the high level area then lower levels of air flow are required.

Disdvantages, supply air has to be relatively warm, no more than 3k lower than the design temp. Air speed must be very low (less that 0.5m/s) so as not to cause uncomfortable drafts.

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Question 10

How do you calculate pressure loss in a duct system.

Answer 10

Delta P equals K time Q squared
K is the system friction constant. Q is the volume. Note that fan size and energy required goes up quickly in proportion to flow rate.

Question 11

How do you calculate static and velocity pressure in a vent system?

Answer 11

Static pressure plus velocity pressure.

Question 12

How do you calculate velocity pressure.

Answer 12

0.6 x velocity squared.

Question 13

What is system characteristic of a ventilation system

Answer 13

System characteristic is the pressure drop across the system as a function of volume flow rate. Delta p equals K x Q squared. K is the constant to the system and Q is the volume flow rate hence efficiency within a given system decreases exponentially in proportion to the volume flow rate.

Question 14

What is fantastic and its relationship to the volume flow rate within a particular system.

Answer 14

Fan characteristic is how much air will be moved as a function of the pressure loss across the ventilation system. This looks like an inverted exponential growth curve so as the pressure drop across the system increases with a given fan the volume flow rate decreases very quickly.

Question 15

What is the operating point of a ventilation system?

Answer 15

It is the point where the fan performance curve meets the system characteristic curve. This determines the volume flow rate through particular system and how much power will be used to move each litre of air

Question 16

What are the properties of forward curve centrifugal fans.

Answer 16

They are 65 to 70% efficient. They have high flow rates. They have low-pressure. They are sensitive to particulate matter and hence they cannot be used in extreme is containing dust. These are the most common fans for commercial applications.

Question 17

What are the properties of Backwards curved centrifugal fans.?

Answer 17

They have approximately 80% efficiency. They have a non overloading power characteristic. They develop high pressure. They handle air streams with low to moderate particulate matter. They tend to be slightly larger forgiven for air flow. They run is about twice the speed of forward Centrifugal fans.

Question 18

Explain the principle of downwards air supply

Answer 18

Air is released from the ceiling mounted diffuser in a horizontal direction. It moves along the ceiling using the Coanda effect and mixes with room air as it goes. Once it has lost its velocity it enters the occupied zone of the room.

Question 19

Explain what isovels are

Answer 19

They are a locus of points in the air jet that have the same air velocity.

Question 20

Explain the term drop with reference to diffusion.

Answer 20

It is the vertical distance from the centre of line of the terminal to the bottom edge of the 0.25m/s isovel.

Question 21

What is the formula for pressure loss in a ductwork system

Answer 21

K*V^2

(K is the system constant and is the total resistance of the system.) V is velocity

Question 22

What are the air speeds in high and low velocity ductwork.

Answer 22

Low is 3.6m/s high is 7.5-15 m/s

Question 23

What is the process for sizing rectangular ductwork?

Answer 23

Calculate the required size of circular ductwork and then use the table that shows the equivalent size of rectangular ductwork. The cross-sectional area of equivalent resistance rectangular ductwork will be greater.

Question 24

What are the advantages of circular and rectangular ductwork

Answer 24

Circular ductwork is cheaper, less noisy, less resistive for a given cross sectional area. Rectangular ductwork has a great cross sectional area for a given height.

Question 25

What are the fan laws? IE the relationship between speed: flow, pressure head, power for a given fan.

Answer 25

For a given fan: flow is proportional to shaft speed (Speed1/Speed2)=(Flow1/Flow2)
Pressure head is equivalent to the square of shaft speed:
(Speed1/Speed2)squared=(pressure1/pressure2)
Power is equivalent to the cube of shaft speed
(Speed1/Speed2)cubed=(power1/power2)