13. 练习1What is the RH of the air at all points on the saturation temperature line
ANS: RH = 100%
What is the dew point at 21° C dry bulb and 50% RH
Dew point = 10° C
What is the Dew Point of air that has an absolute humidity (moisture content) of 10 g/kg
Dew point = 14 oC
What is the RH of the air at 30° C dry bulb and a dew point temperature of 5° C
RH = 20.3%
14. 练习1 (续一)What is the moisture content of the air at 22 ° C dry bulb and 50% RH
Moisture content = 8.4 g/kg
What is the RH at 26 ° c dry bulb and 17 ° c wet bulb
Rh = 40%
What is the wet bulb temperature of the air at 20 °C dry bulb and 50% RH
Wet bulb temperature = 14 °C
What is the RH at 22°C dry bulb and 15°C wet bulb
RH = 47%
15. 练习1 (续二)What is the wet bulb temperature of the air at 24 °C dry bulb and 40% RH
Wet bulb temperature = 15.5 °C
Air at 24 °C dry bulb and 20 °C wet bulb is cooled to 8°C along the saturation curve and then reheated to 21 ° c. What is the rh of the air at this condition
RH = 42%
30. 常用风门Dampers used in HVAC systems are typically air flow throttling devices and can be either modulating or two position. So it have two major type of damper actuator.
They usually have multiple blades mounted on parallel shafts which are rotated in unison by an external actuator to change the blades from open to closed or closed to open
Depending on the requirements of the system the assembly can be arranged as “normally open” or “normally closed” by means of the arrangement of the actuator device used to drive the damper
31. Depending on how the adjacent shafts are linked together the damper assembly is either :-
A parallel blade damper
An opposed blade damper
Parallel blade dampers are constructed so that each blade rotates in parallel with or in the same direction as the blade next to it
Opposed blade dampers are constructed so that adjacent blades rotate opposite to each other
The damper rotation increases the resistance to the air flow since the air is funnelled through a smaller opening
An opposed blade damper must be open further than a parallel damper to obtain the same resistance to air flow常用风门
32. 常用风门The following diagram shows a basic parallel blade damper assembly together with the air flow pattern with the damper at 33% closed
33. 常用风门The following diagram shows a basic opposed blade damper assembly together with the air flow pattern with the damper at 33% closed
34. 常用风门Round or butterfly dampers are typically used to control air flows in ducts that have high static pressure and high velocity characteristics
35. 常用风门Fire dampers are used to prevent superheated air, gases or flames from crossing a fire barrier. They can be duct mounted or installed directly in a fire barrier
They are always two position devices
36. 通风机 离心式
前曲式
后曲式
径向式
轴流式
37. 离心风机CENTRIFUGAL FANS
Found in supply air applications and quietly move air at moderate pressures
Utilise VSD drives for capacity control
Older installations may have fans with inlet guide vanes to vary air flow
Almost always belt driven and not normally rated to operate in smoke extract applications
CENTRIFUGAL FANS CAN HAVE EITHER :-
Forward inclined blades
Backward inclined blades
38. Centrifugal fans with forward inclined blades
Used in the majority of HVAC installations as supply air fans
Used from small domestic applications to large commercial applications in multi story buildings
Centrifugal fans with backward inclined blades
Generally larger and quieter in operation than forward inclined bladed fans
Should always be used in variable volume systems
More expensive to manufacture than forward inclined bladed fans and therefore contractors are reluctant to use them
离心风机
39. 典型离心风机A TYPICAL CENTRIFUGAL FAN WITH BACKWARD INCLINED BLADES IS DETAILED BELOW :-
40. AXIAL FANS
Generally used for moving large volumes of air at low pressures
Usually directly driven and may use vsd drives for capacity contriol
Used in extract, return air, cooling tower and smoke control applications where high noise levels are not an issue
THE MOST COMMON TYPE OF AXIAL FANS USED IN THE HVAC INDUSTRY ARE :-
Propeller fan
Tube axial fan
Vane axial fan轴流风机
41. Propeller Fans are low pressure high air flow units which work up to a maximum static pressure of 19 pa.
Tube axial fans are heavy duty units arranged for installation in ducts. It work up to maximum static pressure of 750 pa.
Vane axial fans are basically tube axial fans with straightening vanes and are quieter in operation than tube axial fans轴流风机
42. CHARACTERISTICS OF AIR FLOW IN DUCTS
A pressure difference (PD) must exist within the duct work to enable air flow to be transferred from the air handling unit fan to the field registers
The PD required must be sufficient to overcome the frictional losses within the duct work and to accelerate the air flow to the required outlet operational velocity
DUCT SYSTEMS HAVE A CERTAIN AMOUNT OF RESISTANCE TO THE FLOW OF AIR
This is known as frictional loss and results from shear forces in the fluid near the duct wall
Frictional loss causes a measurable loss of static pressure in the duct
Frictional loss is proportional to velocity squared
Double the flow rate and four times the pressure loss will occur
风管中气流的特性参数
43. The following fan affinity laws describe what happens to the flow rate, pressure and motor power when one parameter of a fan system changes风管中气流的特性参数
44. A typical curve of air flow rate against static pressure in a duct is detailed below :-风管中气流的特性参数
45.
In HVAC systems the air supplied by the fan includes two types of pressure
Velocity pressure
Static pressure
Velocity PressureThis is the pressure exerted by a moving fluid on an fluid on an object in the path and is kinetic energy. It is proportional to the flow or velocity squared and is measured in Pascals (Pa)
In an air flow system the relationship between velocity pressure and velocity, which is measured in metres / second (m/s) is:-
风管中气流的特性参数
46. If velocity and duct size are known then volume of air flow can be determined from the formula :-
Q = AV
“Q” = air flow rate in cubic metres / second
“A” = cross sectional area of duct in square metres
“V” = velocity of the air flow rate in metres / second
Static pressure
This is the pressure exerted by a stationary fluid perpendicularly on a duct or pipe wall and is potential energy
It is expressed in Pascals (Pa) although non metric measures for static pressure are still common within the HVAC industry (inches water column)
Total pressure
This is the sum of velocity pressure and static pressure and is also measured in Pascals (Pa)风管中气流的特性参数
47. The relationship between velocity, static and total pressure for a duct system with a positive static pressure value is depicted below风管中气流的特性参数