Heating and Air Conditioning I

1. Heating and Air Conditioning I Principles of Heating, Ventilating and Air Conditioning R.H. Howell, H.J. Sauer, and W.J. Coad ASHRAE, 2005 basic textbook/reference material For ME 421 John P. Renie Adjunct Professor – Spring 2009

2. Chapter 9 – Duct and Pipe Sizing • Fans. • Introduction • Air pump – machine that creates pressure difference and causes air to flow – impeller imparts static and kinetic energy to air • Classification - Centrifugal

3. Chapter 9 – Duct and Pipe Sizing • Classification - Centrifugal

4. Chapter 9 – Duct and Pipe Sizing • Fans. • Principles of Operation • The centrifugal force created by rotating the air column enclosed between the blades • The kinetic energy imparted to the air by its velocity leaving the impeller – combination of rotative velocity on the impellor and air speed relative to the impeller • Forward versus backward facing blades – backward facing more efficient (see diagram – Figure 9-3) • Axial flow fans produce all static pressure from the change of velocity of the air passing through the impeller • Propeller fans – small hub-to-tip ratio • Tube-axial – reduced running clearance, higher tip speeds • Vaneaxial fans – tube-axial with guide vanes that give improve pressure characteristics and efficiency.

5. Chapter 9 – Duct and Pipe Sizing • Fans. • Definitions

6. Chapter 9 – Duct and Pipe Sizing • Fans. • Definitions

7. Chapter 9 – Duct and Pipe Sizing • Fans. • Fan Testing • The pilot tube duct traverse can be used to explain the procedure by which a constant speed fan performance curve can be obtained

8. Chapter 9 – Duct and Pipe Sizing • Fans. • Fan Testing • Tested from shut-off to free delivery – flow smoothing • Similarity in fan performance (dynamically) • Fan Laws • Geometric similarity – Fan size D, Rotational speed, N, gas density r, volume flow rate Q, static or total pressure, power H, and mechanical efficiency, h1 • Fan laws mathematically express the facts that when two fans are both members of a geometrically similar series, their performance curves are homologous • At the same point of rating, efficiencies are equal (stated percent of free delivery airflow, or same static to total pressure ratio) • Based on standard air density of 0.075 lb/ft3 – with constant size and speed, power and pressure varies directly as the ratio of the gas density to standard.

9. Chapter 9 – Duct and Pipe Sizing • Fans. • Fan Laws

10. Chapter 9 – Duct and Pipe Sizing • Fans. • Fan Laws – example of performance curve

11. Chapter 9 – Duct and Pipe Sizing • Fans. • Fan Laws – example of performance curve • Bottom curve is the base curve (N = 600 rpm) • Point E (at N = 650 rpm) is determined from point D

12. Chapter 9 – Duct and Pipe Sizing • Fans. • Duct System Characteristics • See Example in Figure 9-3 • Elbows represent resistance – a given flow rate through the system requires a given total pressure – total pressure varies as volume flow rate squared. (turbulent flow only)

13. Chapter 9 – Duct and Pipe Sizing • Fans. • Duct System Characteristics • This allows plotting of Dp curve from one known operating point • Looking at two modifications to System A (Figure 9-7) • Add resistance (Figure 9-9) • Remove resistance (Figure 9-10)

14. Chapter 9 – Duct and Pipe Sizing • Fans. • Fan Selection • Once the Dp of the system is determined, then the fan can be selected to meet the system’s requirement. • Fan manufacturer’s curves or multi-rating tables (at fixed speed) • The point of operation selected must be at a desirable point on the fan curve, maximum efficiency and resistance to stall and pulsation can be attained.

15. Chapter 9 – Duct and Pipe Sizing • Fans. • Fan Selection

16. Chapter 9 – Duct and Pipe Sizing • Fans. • Fan Selection

17. Chapter 9 – Duct and Pipe Sizing • Fans. • Fan Selection – multi-rating Table 9-11b

18. Chapter 9 – Duct and Pipe Sizing • Fans. • Fan Selection

19. Chapter 9 – Duct and Pipe Sizing • Air Diffusing Equipment. • General • Supply and diffusing equipment as well as return and exhaust equipment. • Acceptable, effective draft temperature (air temperature, air motion, relative humidity, and their physiological effects on the human) • The feeling of coolness by air motion

20. Chapter 9 – Duct and Pipe Sizing • Air Diffusing Equipment. • General • Conditioned air is usually supplied to air outlets at velocities much higher than would be acceptable in the occupied zone. • Proper air distribution call for • Entrainment of rom air by the primary airstream outside the zone of occupancy so that air motion and temperature differences are reduced to acceptable limits before air enters the occupied zone • Counteraction of the natural convection and radiation effects within the room. • Supply Air Outlets – types – see selection procedure (9.3.2)

21. Chapter 9 – Duct and Pipe Sizing • Air Diffusing Equipment. • Noise Control • Sound at outlet is function of air velocity and transmission of systemic noise • See Examples 9.3 and 9.4 for outlet “throw” and “draftiness” • Pipe, Tube, and Fittings • Selection in Table 9-8 and 9-9 – common sizes and dimensions • Materials – Table 9-10 is guide to material used in HVAC • Codes • Plastic pipes – popularity (cost, flexibility, non-corrosive) • Thermoplastic • Thermoset • Pipe Fittings – elbows, tees, coupling, reducers, unions • Pipe Sizing (see Example 9.5)

22. Chapter 9 – Duct and Pipe Sizing

23. Chapter 9 – Duct and Pipe Sizing

24. Chapter 9 – Duct and Pipe Sizing

25. Chapter 9 – Duct and Pipe Sizing

26. Chapter 9 – Duct and Pipe Sizing

27. Chapter 9 – Duct and Pipe Sizing

28. Chapter 9 – Duct and Pipe Sizing

29. Chapter 9 – Duct and Pipe Sizing

30. Chapter 9 – Duct and Pipe Sizing

31. Chapter 9 – Duct and Pipe Sizing

32. Chapter 9 – Duct and Pipe Sizing • Pumps • Classification • Reciprocating • Rotary • Centrifugal • First two are positive displacement – fixed amount discharged for a fixed speed. • Centrifugal are more like fans with impeller • Fluid enters the eye of the impeller and energy is added to the liquid by the impeller. The casing collects the liquid as it leaves the impeller and guides it out the discharge of the pump. • Pump Laws – for CPs – Table 9-11 • Pump Performance Curve • See Figure 9-13 • Maximum efficiency is not always the most important factor • Quiet operation, durability, simple service, minimum maintenance

33. Chapter 9 – Duct and Pipe Sizing

34. Chapter 9 – Duct and Pipe Sizing • Pumps

35. Chapter 9 – Duct and Pipe Sizing • Pumps – Hydronic Systems • Loop type – open/closed • Full flow or throttling flow • System head curve

36. Chapter 9 – Duct and Pipe Sizing • Pumps - Hydronic Systems • Independent head and system friction head • Latter dependent on total system flow