VIEWS: 474 PAGES: 44 CATEGORY: Technology POSTED ON: 7/9/2010 Public Domain
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air that must supplied and the condition it must have to remove given amounts of energy and water from the space to be withdrawn at a specified condition.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