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					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
    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.
    Chapter 3 – Basic HVAC Calculations
• Applying Thermodynamics to HVAC Processes
    Chapter 3 – Basic HVAC Calculations
• Applying Thermodynamics to HVAC Processes
    Chapter 3 – Basic HVAC Calculations
• Applying Thermodynamics to HVAC Processes
     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     energy and water from the space to be withdrawn at a specified condition.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
     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
     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
     Chapter 3 – Basic HVAC Calculations
• Cooling and Dehumidifying Air
     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.
     Chapter 3 – Basic HVAC Calculations
• Heating and Humidifying Air
     Chapter 3 – Basic HVAC Calculations
• Adiabatic Mixing of Two Streams of Air
     Chapter 3 – Basic HVAC Calculations
• Adiabatic Mixing of Moist Air with Injected Water
    Chapter 3 – Basic HVAC Calculations
• Moving Air
    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 101.325 kPa (68 oF and 14.7 psia)
       • Density is 1.204 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
     Chapter 3 – Basic HVAC Calculations
• Sensible heat gain corresponding to the change of dry-bulb
  temperature for a given airflow (at standard conditions)
     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.
     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.
     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).
    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.
     Chapter 3 – Basic HVAC Calculations
• Single-Path Systems - schematic
     Chapter 3 – Basic HVAC Calculations
• Single-Path Systems – equations for supply air
     Chapter 3 – Basic HVAC Calculations
• Single-Path Systems – equations for supply air
      Chapter 3 – Basic HVAC Calculations
•   Single-Path Systems – supply air for ventilation
     1. Supply air for ventilation – needed when the amount of outside air is
        not adequate
     2. 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
     3. Desired air exchange rate not satisfied – supply air is determined
     4. 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.
    Chapter 3 – Basic HVAC Calculations
• Single-Path Systems – Example Problem 3-3
    Chapter 3 – Basic HVAC Calculations
• Single-Path Systems – Example Problem 3-3
    Chapter 3 – Basic HVAC Calculations
• Single-Path Systems – Example Problem 3-3
    Chapter 3 – Basic HVAC Calculations
• Single-Path Systems – Example Problem 3-3
    Chapter 3 – Basic HVAC Calculations
• Single-Path Systems – Cycle Diagram
       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
    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.
    Chapter 3 – Basic HVAC Calculations
• Single-Path Systems – Cycle Diagram
   • Room (internal) sensible loads which determine the CFMs consist of:
    Chapter 3 – Basic HVAC Calculations
• Single-Path Systems – Cycle Diagram
     Chapter 3 – Basic HVAC Calculations
• Single-Path Systems – Psychrometric Representation
     Chapter 3 – Basic HVAC Calculations
• Single-Path Systems – Psychrometric Representation
     Chapter 3 – Basic HVAC Calculations
• Single-Path System - Psychrometric Representation
     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
     Chapter 3 – Basic HVAC Calculations
• Single-Path System – Final Example
     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
     Chapter 3 – Basic HVAC Calculations
• Single-Path System – Final Example
     Chapter 3 – Basic HVAC Calculations
• Single-Path System – Final Example
     Chapter 3 – Basic HVAC Calculations
• Single-Path System – Final Example
     Chapter 3 – Basic HVAC Calculations
• Single-Path System – Final Example
     Chapter 3 – Basic HVAC Calculations
• Single-Path System – Final Example

				
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