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 3 – Basic HVAC Calculations
Applying Thermodynamics to HVAC Processes
Looking at a simplified (but complete) air-conditioning system
Terminology: qsensible, mwater, qL, hw, solar gains
Look at first law of thermodynamics (energy) and conservation of mass
Air is removed from the room, returned to the air-conditioning apparatus where it is reconditioned, and then supplied again to the room.
Many cases, it is mixed with outside air required for ventilation
Outdoor air (o) is mixed with return air (r) from the room and enters the apparatus at condition (m)
Air flows through the conditioner and is supplied to the space (s).
The air supplied to the space absorbs heat qs and moisture mw, and the cycle continues.

3. Chapter 3 – Basic HVAC Calculations
Applying Thermodynamics to HVAC Processes

4. Chapter 3 – Basic HVAC Calculations
Applying Thermodynamics to HVAC Processes

5. Chapter 3 – Basic HVAC Calculations
Applying Thermodynamics to HVAC Processes

6. Chapter 3 – Basic HVAC Calculations
Absorption of Space Heat and Moisture Gains
AC usually reduces to determining the quantity of moist air that must supplied and the condition it must have to remove given amounts of energy and water from the space to be withdrawn at a specified condition.
Sensible heat gain – addition of energy only – not wrt water

7. Chapter 3 – Basic HVAC Calculations
Heating or Cooling of Air – without moisture gain or loss – straight line on psychrometric chart since humidity ratio is constant

8. Chapter 3 – Basic HVAC Calculations
Cooling and Dehumidifying Air
Moist air brought down below its dew point temperature – some of the water will condense and leaves the air stream
Assume condensed water is cooled to the final air temperature before draining from the system

9. Chapter 3 – Basic HVAC Calculations
Cooling and Dehumidifying Air

10. Chapter 3 – Basic HVAC Calculations
Cooling and Dehumidifying Air
Moist air brought down below its dew point temperature – some of the water will condense and leaves the air stream
Assume condensed water is cooled to the final air temperature before draining from the system
Cooling and dehumidifying process involves both sensible heat transfer and latent heat transfer where sensible heat transfer is associated with the decrease in dry-bulb temperature and the latent heat transfer is associated with the decrease in humidity ratio.

11. Chapter 3 – Basic HVAC Calculations
Heating and Humidifying Air

12. Chapter 3 – Basic HVAC Calculations
Adiabatic Mixing of Two Streams of Air

13. Chapter 3 – Basic HVAC Calculations
Adiabatic Mixing of Moist Air with Injected Water

14. Chapter 3 – Basic HVAC Calculations
Moving Air

15. Chapter 3 – Basic HVAC Calculations
Approximate Equations Using Volume Flow Rates
Since volumes of air change – need to make calculations with mass of dry air instead of volume. But volumetric flow rates define selection of fans, ducts, coils, etc.
Use volume while still considering mass by using volume rates based on standard air conditions
Dry air at 20 oC and kPa (68 oF and 14.7 psia)
Density is kg/m3 (0.075 lb/ft3) – dry air
Specific volume is 0.83 m3/kg (13.3 ft3/lb) – dry air
Saturated air at 15 oC has about same density and volume
Need to convert actual volumetric flow conditions to standard
Say you need 1,000 cfm outside air rate at standard conditions
Outside measured at 35 oC dry bulb and 23.8 oC wet bulb corresponding to a specific volume of 14.3 ft3/lb.
Then, the actual flow rate would be 1,000 (14.3/13.3) = 1,080 cfm
1,000/13.3 = 1,080/14.3 = mass rate (lb/min) of moist air

16. Chapter 3 – Basic HVAC Calculations
Sensible heat gain corresponding to the change of dry-bulb temperature for a given airflow (at standard conditions)

17. Chapter 3 – Basic HVAC Calculations
Latent heat gain corresponding to the change of humidity ratio W for a given airflow (at standard conditions).
The latent heat gain in Watts (Btu/h) as a result of a difference in humidity ratio DW between the incoming and leaving air flowing at standard conditions.

18. Chapter 3 – Basic HVAC Calculations
Total heat gain corresponding to the change of dry-bulb temperature and humidity ratio W for a given airflow (at standard conditions).
The total heat gain in Watts (Btu/h) as a result of a difference in enthalpy Dh between the incoming and leaving air flowing at standard conditions.

19. Chapter 3 – Basic HVAC Calculations
Total heat gain corresponding to the change of dry-bulb temperature and humidity ratio W for a given airflow (at standard conditions).

20. Chapter 3 – Basic HVAC Calculations
Single-Path Systems
Simplest form of all-air HVAC system serving a single temperature control zone
Responds to one set of space conditions, where conditions vary uniformly and the load is stable.
Schematic of system – return fan necessary under certain conditions of Dp.
Need for reheat – necessary to control humidity independent of the temperature requirements.
Equations for single-path systems – air supplied must be adequate to take care of each room’s peak load conditions. Peak loads may be governed by sensible or latent room cooling loads, heating loads, outdoor air requirements, air motion, and exhaust. – let us look at each of these loads and what air volume is required to satisfy these demands.

21. Chapter 3 – Basic HVAC Calculations
Single-Path Systems - schematic

22. Chapter 3 – Basic HVAC Calculations
Single-Path Systems – equations for supply air

23. Chapter 3 – Basic HVAC Calculations
Single-Path Systems – equations for supply air

24. Chapter 3 – Basic HVAC Calculations
Single-Path Systems – supply air for ventilation
Supply air for ventilation – needed when the amount of outside air is not adequate
Supply air not adequate for the amount of exhaust makeup required – no return air comes from the room and entire volume of make-up ventilation air becomes an outside air burden to system
Desired air exchange rate not satisfied – supply air is determined
Desired air movement not satisfied, based on area index parameter, K.
Each of the above conditions are used at different times – Case 1 when outside air governs, Cases 3 and 4 when air movement governs, and Case 2 when exhaust governs.

25. Chapter 3 – Basic HVAC Calculations
Single-Path Systems – Example Problem 3-3

26. Chapter 3 – Basic HVAC Calculations
Single-Path Systems – Example Problem 3-3

27. Chapter 3 – Basic HVAC Calculations
Single-Path Systems – Example Problem 3-3

28. Chapter 3 – Basic HVAC Calculations
Single-Path Systems – Example Problem 3-3

29. Chapter 3 – Basic HVAC Calculations
Single-Path Systems – Cycle Diagram

30. Chapter 3 – Basic HVAC Calculations
Single-Path Systems – Cycle Diagram
Each state point is identified both in summer and winter
Change of Dt is result of sensible heat loss or gain, qS
Change in DW is result of latent heat loss or gain, gL
All return air is assumed to pass from the room through a hung-ceiling return air plenum
Supply air CFMS at the fan discharge temperature tsf (summer mode) absorbs the transmitted supply duct heat qsd and supply air fan velocity pressure energy qsf,vp thereby raising the temperature to ts

31. Chapter 3 – Basic HVAC Calculations
Single-Path Systems – Cycle Diagram
Room supply air absorbs room sensible and latent heat qSR and qLR along the room sensible heat factor (SHR) line s-R, reaching the desired room state, tR and WR.

32. Chapter 3 – Basic HVAC Calculations
Single-Path Systems – Cycle Diagram
Room (internal) sensible loads which determine the CFMs consist of:

33. Chapter 3 – Basic HVAC Calculations
Single-Path Systems – Cycle Diagram

34. Chapter 3 – Basic HVAC Calculations
Single-Path Systems – Psychrometric Representation

35. Chapter 3 – Basic HVAC Calculations
Single-Path Systems – Psychrometric Representation

36. Chapter 3 – Basic HVAC Calculations
Single-Path System - Psychrometric Representation

37. Chapter 3 – Basic HVAC Calculations
Single-Path System – Sensible Heat Factor (Ratio)
Sensible heat factor (ratio), SHF or SHR, is the ratio of sensible heat for a process to the total of sensible and latent heat for the process.
The sensible and latent combined is referred to as the total heat
On psychrometric chart, the protractor provides this ratio and may be used to establish the process line for changes in the conditions of the air across the room or the conditioner on the chart
The supply air to a conditioned space must have the capabilty to offset both the room’s sensible and latent heat loads.
Connecting the room and supply points with a straight line provides the sensible heat factor condition. The conditioner provides the simultaneous cooling and dehumidifying that occurs.
Horizontal line would be SHF = 0.0 (only sensible)
Line with SHF = 0.5 would be half sensible and half latent

38. Chapter 3 – Basic HVAC Calculations
Single-Path System – Final Example

39. Chapter 3 – Basic HVAC Calculations
Single-Path System – Final Example
Sensible and latent loads given
Room Conditions: (75 oF and 55% RH) – Supply at 58 oF
Outside Conditions: 96 oF DB, 77 oF WB and 20% of total flow

40. Chapter 3 – Basic HVAC Calculations
Single-Path System – Final Example

41. Chapter 3 – Basic HVAC Calculations
Single-Path System – Final Example

42. Chapter 3 – Basic HVAC Calculations
Single-Path System – Final Example

43. Chapter 3 – Basic HVAC Calculations
Single-Path System – Final Example

44. Chapter 3 – Basic HVAC Calculations
Single-Path System – Final Example