HVAC Systems Primary Functions

Transcription

1 HVAC Systems Primary Functions Temperature Control Cooling Heating Humidity Control Humidification Dehumidification Air Quality Control Ventilation Cleaning

2 HVAC Systems Energy Considerations Typical Energy Use in a Commercial Building

3 HVAC Systems Efficient i Use of Energy Requirements Optimum Energy Designs Well-Developed Energy Use Policies Dedicated Management backed up by Properly Trained and Motivated Operating Staff

4 HVAC Systems Minimum i Guidelines in Energy Conservation, Design and Operation ASHRAE Standard , Energy Efficient Design of New Buildings Except Low-Rise Residential Buildings ASHRAE Standard , Energy Conservation in Existing Buildings

5 HVAC Systems Typical Building Design Heat Losses or Gains

6 HVAC Systems Some Relevant Energy/Emissions Statistics Buildings account for: 39% of all the energy (36%) 71 % of all the electricity (66%) 12% of water consumption used in the US Emissions related to building energy use account for 40% of non-industrial waste 38% of CO 2 emissions (35%) >47 % of SO 2 emissions >22% of NO x emissions

7 HVAC Systems Annual Energy Use Per Unit Floor Area

8 HVAC Systems 10 years ago Capital Cost Estimating Factors

9 Building Costs Energy Costs

10 HVAC Systems Schematic of a Typical Commercial Air-Conditioning System

11 HVAC Systems Elementary Air Temperature Control System

12 HVAC Systems Air Handler and Associated Controls for Simple Constant-Volume, Single-Duct All-Air System

13 HVAC Systems Schematic of a Blow-Through Air Handler With Hot and Cold Decks and Zone Dampers

14 HVAC Systems Simplified Control Schematic for a Constant-Volume reheat System

15 HVAC Systems Simplified Control Schematic of a Single-Duct VAV System

16 HVAC Systems Simplified Control Schematic of a Dual-Duct System

17 HVAC Systems Multi-Zone System With Hot and Cold Plenum Reset

18 HVAC Systems Air-Water Induction Unit From Central A/C Unit Typically installed at perimeter wall under window or overhead Hot or Chilled Water

19 HVAC Systems Typical Fan-Coil CilUit Unit Air-Conditioned Air Chilled Water or Brine/ Hot Water or Steam or Electric Heater Recycles Room Air, Cheapest Perimeter System, Ventilation Provided Separately

20 HVAC Systems Typical Air-Conditioning Ventilator with Separate Coils

21 HVAC Systems Schematic View of a Room Air-Conditioner

22 Residential Cooling and Heating Loads Distinguishing features from other buildings Smaller Internal Heat Gains heat gain or loss through structural components air leakage or ventilation small internal heat gains (occupants, lights) Varied Use of Spaces flexible localized or temporary temperature excursions tolerable Fewer Zones. single or few zones - one thermostat Capacity cannot be redistributed as loads change over day Greater Distribution Losses. ducts are installed in unconditioned buffer spaces require significant increase in unit capacity distribution ib ti gains/losses cannot be neglected

23 Residential Cooling and Heating Loads Distinguishing features from other buildings Partial Loads systems use units of small capacity ~ 12,000 to 60,000 Btu/h cooling ~ 40,000 to 120,000 Btu/h heating units mostly operate at partial load oversized units are bad for system performance (especially for cooling in areas of high WBT) Dehumidification Issues dehumidification only when cooling unit operates space condition control is driven by room thermostats (sensible heat actuated) excessive sensible capacity leads to short-cycling and degraded dehumidification

24 Residential Cooling and Heating Loads Classification based on load profiles Single-Family Detached Exposed Walls in four directions Single l zone one thermostat Two-story houses may have separate cooling systems per floor Multifamily Other Exposed Walls not in four directions (east/west exposure plays a role)

25 Residential Cooling and Heating Loads Approaches Heating No solar or internal gains and no heat storage Heat losses assumed instantaneous Cooling Need to take account of temperature swing via empirical data and models Non-residential methods lead to unrealistically high loads Use Residential Load Factor (RLF) method From detailed residential heat balance (RHB) of prototyped buildings over a range of climates.

26 Residential Heating Load Considerations No solar, internal gains, heat storage (highest load during early am) Heat losses assumed instantaneous Calculated for a normal worst case condition (indoor/outdoor design conditions, ventilation/infiltration) Estimate maximum probable heat loss per room Transmission Losses (walls, floor, roof/ceiling, fenestration/doors) Infiltration & Ventilation If night thermostat set-back is used, may need excess capacity.

27 Residential Heating Load Procedure Outdoor design condition (temp., wind speed and dir.) Indoor design condition (temp., humidity level) Temps. of adjacent unconditioned dspaces; Ground temp. if below grade Estimate overall heat transfer coefficients for every boundary element Estimate area of each boundary element Compute heat transmission losses (Table 6-17) Estimate infiltration and compute associated energy Estimate required ventilation and compute associated energy Calculate the total heating load Estimate pickup loads for intermittently heated buildings or thermostat set-back.

28 Residential Heating Load Equations Heating Load Factor

29 Residential Heating Load Below-Grade Surfaces Heating Load Factor

30 Residential Heating Load Below-Grade Surfaces Basement Walls

31 Residential Heating Load Below-Grade Surfaces Basement Floors

32 Residential Heating Load On-Grade Surfaces Concrete Slab Floors Unheated Heated Heating Load Factor

33 Residential Heating Load On-Grade Surfaces Heat Loss Coefficient

34 Residential Heating Load Infiltration Heat Losses Sensible Latent t

35 On Infiltration Estimation Methods Air Change Method Simple Highly Empirical (performance of similar construction)

36 On Infiltration Estimation Methods Crack Method Requires estimation of indoor-outdoor pressure differences Wind Effect Stack Effect Pressurization Requires estimation of building envelope permeability and associated crack characteristics.

37 On Infiltration Estimation Methods Estimation based on Effective Leakage Area

38 Design Conditions Outdoor Weather Data Figure 4-4 Climatic Design Information Table 4-7 Design Conditions by location Information is provided on two levels Annual seasonal means Monthly means (to include seasonal variation) Often data is given in association with percentiles Warm Season 0.4, 1, 2 annual percentiles Cold Season 99.6, 99 annual percentiles , 2, 5, monthly percentiles Variable value at n% means that the value is equaled ed or exceeded n% of the time.

39 Design Conditions Outdoor Annual Heating and Humidification Design Conditions Annual Cooling, Dehumidification, and Enthalpy Design Conditions Extreme Annual Design Conditions Monthly Design Conditions Temperatures, Degree-Days, and Degree-Hours Monthly Design Dry-Bulb, Wet-Bulb, and Mean Coincident Temperatures Mean Daily Temperature Range Clear-Sky Solar Irradiance

40 Design Conditions Annual Heating and Humidification Coldest Month (1=January) Heating, 99.6% and 99%: Dry-Bulb Temperature (DB) Humidification, 99.6% and 99%: Dew Point (DP) For Humidification Humidity Ratio (HR) Decisions Mean Coincident Dry Bulb Temperature (MCDB) Coldest Month, 0.4%, 1%: Wind Speed (WS) - mph Mean Coincident Dry Bulb Temperature (MCDB) For the 99.6% 6%DB value Mean Coincident Wind Speed (MCWS) - mph Prevailing Coincident Wind Direction (PCWD) Maximum Heating Load To Size Equipment Peak Loads accounting for Infiltration

41 Design Conditions Annual Cooling, Dehumidification, and Enthalpy Time that Maximum Sensible Hottest Month (1=January) Cooling Load occurs DB Range For Cooling Load Cooling, 0.4%, 1%, 2%: DB and Mean Coincident Wet Bulb Temperature (MCDB) Evaporation, 0.4%, 1%, 2%: Wet Bulb Temperature (WB)andMCDB Sizing Chillers & Air-Conditioners For the 0.4 % DB value MCWS and PCWD Design of Cooling Towers, Evaporative Coolers, Fresh-Air Ventilation Systems Estimates of Peak Loads accounting for Infiltration

42 Design Conditions Annual Cooling, Dehumidification, and Enthalpy Dehumidification, 0.4%, 1%, 2%: DP, HR, MCDB Enthalpy, 0.4%, 1%, 2%: Enthalpy (Btu/lb) and MCDB Humidity Control lapplications Desiccant Dehumidification, Cooling-based Dehumidification Fresh-Air Ventilation Systems System Analysis at Partial-Load Conditions Useful for Cooling Load related to Infiltration and/or Ventilation Number of Hours between 8am and 4pm with 55F<DB<69F

43 Design Conditions Extreme Annual Design Conditions Extreme Annual WS, 1%, 2.5%, 5% Extreme Maximum WB Used for Smoke Management Systems DB and Mean Coincident Wet Bulb Temperature (MCDB) Extreme Annual DB Mean & Standard Deviation Minima & Maxima n-year Return Period Values of Extreme DB n=5, 10, 20, 50 years Minima & Maxima

44 Design Conditions Monthly Design Conditions Annual and Monthly Data Average Temperatures (Tavg) and associated Standard Deviations (Sd) Heating Degree Days, for 50F (HDD50) and 65 F (HDD65) bases Cooling Degree Days, for 50F (CDD50) and 65 F (CDD65) bases Degree Hours, for 74F (CDH74) and 80F (CDD80) bases Used for Energy Estimation

45 Design Conditions Monthly Design Conditions Annual and Monthly Data DB and MCWB; at 0.4%, 2%, 5%, 10% WB and MCDB; at 0.4%, 2%, 5%, 10% Mean Daily Temperature Range: Mean Dry Bulb Range (MDBR) Mean Coincident Dry Bulb Range (MCDBR) and Wet Bulb Range (MCWBR) for 5% DB and 5% WB Clear Sky Solar Irradiance Beam (taub)&diffuse(taud) (taud) Irradiance Optical Depths Beam Normal (Ebn, noon) & Diffuse Horizontal (Edh, noon) Irradiances at Solar Noon

46 Design Conditions Comments Design values based on DB temperature relate to peak sensible outdoor component Design values based on WB temperature relate to enthalpy of outdoor air Conditions based on DP relate to peaks of humidity ratio Designer must decide which set(s) of conditions and probability of occurrence (expressed by the percentiles) apply to the design situation in hand.

47 Design Conditions Comments - Heating Minimum Temperatures usually occur between solar 6:00am-8:00am For continuous occupancy the recommended DB design temperatures should be used For occupancy predominantly during the middle of the day may use DB temperatures above the recommended minimum

48 Design Conditions Comments - Cooling Maximum Temperatures usually occur between solar 2:00pm- 4:00pm Design DB and MCWB temperatures should be used for building cooling loads For continuous o occupancy the recommended design temperatures should be used For occupancy predominantly during the middle of the day may use temperatures below the recommended maximum Peak occupancy load may occur before the effect of the maximum temperature is felt Peak occupancy load may occur during months other than the ones during which the maximum temperature is expected.

49 Indoor Design Conditions Comfort and Health Physiological Principles Perception of the Environment often subjective Sick Building Syndrome (SBS) Oi Origin i not always obvious Irritation of mucus membranes, fatigue, headache, lower resp. symptoms, nausea, nosebleeds, chest tightness, fever Building-Related Illness (BRI) Known origins i Bioaerosols humidifier fever, asthma, allergies

50 Contaminants Air Quality Body Temperature Internal 98.6+/-1F Indoor Design Conditions Comfort and Health Physiological Principles Skin Temperature varies 88F to 96.8F under normal conditions 91.5F +/-2.5F typically for comfort. Moisture/Humidity level Static Electricity Prevention and Treatment of Disease (50% highest mortality of some organisms) Mold and bacteria growth due to Visible and Concealed condensation

51 Indoor Design Conditions Body s Interaction with the Environment

52 Indoor Design Conditions Environmental Indices

53 Indoor Design Conditions Mean Radiant Temperature Definition Simplification

54 Definition Indoor Design Conditions Operative Temperature Simplification t 0 is the operative temperature t a is the operative temperature

55 Indoor Design Conditions Comfort Chart

56 Metabolic Rate Indoor Design Conditions Other Factors and Conditions Clothing Level