12 Cooling Load Calculations

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Cooling Load Module

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12 Cooling Load Calculations

  1. 1. Cooling Load
  2. 2. Contents • Principle of cooling load • Why cooling load & heat gains are different • Design conditions • Understand CLTD/CLF method • An example
  3. 3. Cooling Load • It is the thermal energy that must be removed from the space in order to maintain the desired comfort conditions • HVAC systems are used to maintain thermal conditions in comfort range
  4. 4. Purpose of Load Estimate • Load profile over a day • Peak load (basis for equipment sizing) • Operation Energy analysis • HVAC Construction cost
  5. 5. Principles of cooling Load Estimate • Enclosure heat transfer characteristics – Conduction – Convection – radiation • Design conditions – Outdoor & indoor • Heat Gains – Internal – External or Solar • Thermal capacity
  6. 6. Space Characteristics • orientation • Size and shape • Construction material • Windows, doors, openings • Surrounding conditions • Ceiling
  7. 7. Space Characteristics • Occupants (activity, number, duration) • Appliances (power, usage) • Air leakage (infiltration or exfiltration) • Lighting (W/m2)
  8. 8. Indoor Design Conditions Basic design parameters • Air temperature – Typically 22-26 C • Air velocity – 0.25 m/s • Relative humidity – 30-70 % • See ASHRAE 55 – 2004 Comfort Zone
  9. 9. Indoor Design Conditions • Indoor air quality – Air contaminants – Air cleaning • Acoustic requirements • Pressurization requirements
  10. 10. Outdoor Design Conditions • Weather data required for load calculation – Temperature & humidity – Wind speed, sky clearness , ground reflectance etc • Design outdoor conditions data can be found in ASHRAE Fundamentals Handbook
  11. 11. Outdoor Design Conditions • ASHRAE Fundamentals 2001 – Design severity based on 0.4%, 1%, & 2% level annually (8760h) – For example at 1% level, the value is exceeded in 0.01x8760h = 87.6 h in a year
  12. 12. Outdoor Design For Cooling Criteria: 0.4% DB and MWB Station Cooling DB/MWB Miri 0.4% 1% 2% Malaysia DB (˚C ) MWB ( DB MWB DB MWB ˚C ) 32.2 26.3 31.8 26.3 31.4 26.2 Source: ASHRAE Fundamentals 2001
  13. 13. Terminology • Space- a volume without partition or a group of rooms • Room- an enclosed space • Zone- a space having similar operating characteristics
  14. 14. Heat Gain • Space Heat gain – The instantaneous rate at which heat enters into , out of, or generated within a space. The components are: Heat gains Convective Radiant (%) • Sensible gain (%) Solar 42 58 • Latent gain radiation with internal shading Fluorescent 50 50 lights People 67 33 External wall 40 60
  15. 15. Heat Gain
  16. 16. Cooling Load • Space Cooling load – The rate at which heat must be removed from a space to maintain air temperature and humidity at the design values • Cooling load differs from the heat gain due to – delay effect of conversion of radiation energy to heat – Thermal storage lag
  17. 17. Heat Gain = Cooling Load
  18. 18. Heat Gain = Cooling Load Thermal storage and Construction Type
  19. 19. Time of the Day: Solar Radiation
  20. 20. Time-delay Effect: Lighting
  21. 21. Extraction Rate • Space Heat extraction rate – The actual heat removal rate by the cooling equipment from the space – The heat extraction rate is equal to cooling load when the space conditions are constant which is rarely true.
  22. 22. Heat Balance The principal terms of heat Gains/Losses are indicated below . (Source: ASHRAE Handbook Fundamentals 2005)
  23. 23. Coil Load • Cooling coil load – The rate at which energy is removed at the cooling coil – Sum of: • Space cooling load (sensible + latent) • Supply system heat gain (fan + supply air duct) • Return system heat gain (return air duct) • Load due to outdoor ventilation rates (or ventilation load)
  24. 24. External Loads 1. Heat gains from Walls and roofs – sensible 2. Solar gains through fenestrations – Sensible 3. Outdoor air – Sensible & latent
  25. 25. Internal Loads 1. People – Sensible & latent 2. Lights – sensible 3. Appliances – Sensible & latent
  26. 26. Total Cooling Load
  27. 27. Cooling Load Components • Space cooling load – Sizing of supply air flow rate, ducts, terminals and diffusers – It is a component of coil load – Bypassed infiltration is a space cooling load • Cooling coil load – Sizing of cooling coil and refrigeration system – Ventilation load is a coil load
  28. 28. Refrigeration Load • The capacity of the refrigeration system to produce the required coil load.
  29. 29. Profiles of Offshore Systems Cooling Loads Components % Load %Load %Load %Load LQ (L) LQ (U) CCR SG/MCC Solar Transmission 3 4 7 4 Occupants 3 3 3 0 Lights 5 5 8 4 Equipment 10 1 29 21 Outdoor air bypassed 7 8 5 6 Outdoor air not 72 79 48 64 bypassed Total 100 100 100 100
  30. 30. Heat Load Components Outdoor air & Electrical Equipment loads (77-85% ) People: 3% Lighting: 4-8% Solar Transmission: 3-7% Infiltration : 5-8%
  31. 31. Calculation Methods 1. Rule of thumb method – Least accurate – eg 100 btu/ft2 for a space 2. Static analysis (Room temperature is constant) – CLTD/CLF method 3. Dynamic analysis – Computer modeling
  32. 32. CLTD/CLF Method • Cooling load is made up of – Radiation and conduction heat gain – Convection heat gain • Convective gain is instantaneous – No delay – Heat gain equals cooling load • Conductive and radiation heat gains are not instantaneous – Thermal delay – Heat gain is not equal to cooling load – Use CLTD & CLF factors
  33. 33. CLTD/CLF Method (ASHRAE 1989) Cooling load due to solar & internal heat gains • Glazing (sensible only) – Radiation & conduction – Convection (instantaneous) • Opaque surface ( wall, floor, roof) load (sensible only) – Conduction – Convection (instantaneous) • Internal loads (sensible & latent) – Radiation & conduction – Convection (instantaneous)
  34. 34. Cooling Load Temperature Difference CLTD Compare Q transmission = UA (T o – T i ) Q transmission = UA (CLTD) • CLTD is theoretical temperature difference defined for each wall/roof to give the same heat load for exposed surfaces to account for the combined effects of radiation, conductive storage, etc – It is affected by orientation, time , latitude, etc – Data published by ASHRAE
  35. 35. Cooling Load Factor (CLF) • This factor applies to radiation heat gain • If radiation is constant, cooling load = radiative gain • If radiation heat is periodical, than Q t = Q daily max (CLF) CLF accounts for the delay before radiative gains becomes a cooling load
  36. 36. Glazing glass • Q = A (SC) (SHGF) (CLF) A= glass area SC= shading coefficient Solar ray SHGF= solar heat gain factor, tabulated by ASHRAE CLF= cooling load factor, tabulated by ASHRAE transmitted • Q = U x A x CLTD reflected U= surface U-factor absorbed A= surface area CLTD= cooling load temperature difference
  37. 37. Opaque Surfaces • Q 2 = UA (CLTD) U= surface U-factor A= surface area CLTD= cooling load temperature difference • Tabulated or chart values for CLTD can be referred • Offshore enclosure – Light weight – Metal frame with insulation – Group G wall with U-value about 0.5-1.0 W/m2 K
  38. 38. CLTD for Sunlit Wall Group G Source: ASHRAE Fundamental
  39. 39. Opaque Surface Calculations • Use Table for wall CLTD • Use Table for roof CLTD – Select wall/roof type – Look up uncorrected CLTD – Correct CLTD CLTD c=(CLTD+LM)+ (25.5-t r) + (t m-29.4) • LM= latitude /month correction (Table ) • T r = indoor temperature (22C) • T m= average temperature on the design day = (35+22)/2 = 28.5 C Eg. If CLTD=40 C, LM=-1.7 (west face) CLTD c= (40-1.7) + (25.5-22)+ (28.5-29.4) = 40.9 C
  40. 40. Types of Internal Load • Internal loads are – People – Lights – Equipment or appliances • Consist of convective and radiant components – Light (mostly radiant) – Electrical heat (radiant and convective) – People (most convective) • Time-delay effect due to thermal storage
  41. 41. Internal Load- Lighting Area Light Power •Heat gain (lighting) Density W/m2 = 1.2 x total wattage x CLF Office 25 Corridor 10 Or based on light power Sleeping 10 density ranging from 10-25 CCR MCC/SG 25 25 W/m2 Kitchen 25 (average density, say=20 Recreation 20 W/m2) •Where light is continuously on, CLF=1
  42. 42. Internal Loads- People • Q people-s = No x sensible heat gain/p x CLF • Q people-L = No x latent heat gain/p
  43. 43. Internal Load – Equipment Heat • Cooling of electrical equipment in MCC/SG is an important function of HVAC system offshore. The components include: • Transformers • Motors • Medium/high voltage switchgears • Cables & trays • Motor starters • Inverters • Battery chargers • Circuit breakers • Unit panel board etc • Heat dissipation from these equipments are mainly based data published by the manufacturers
  44. 44. Typical Outdoor & Indoor Design Conditions Used Here Conditions Dry-bulb % RH Moisture content, temperature (C) kg/kg Outdoor air 35 70 0.025 Indoor air 22 55 0.009 Difference 13 0.016 ASHRAE fundamental Handbook published data, at 0.4%, 1% and 2% design level. At 0.4% design level, Miri has only 35h (out of 8760 h a year) at 32.2 DB & 26.3 WB or higher
  45. 45. Infiltration Air is Cooling Load • Load due to Ventilation air into the space Sensible load, (W) = mass flow rate x specific heat x (∆T) = 1.23 x l/s x (To – T i) or (1.08 x cfm x ∆T) Where To = Outside temperature, C Ti = indoor air temperature, C
  46. 46. Ventilation Cooling Load Ventilation latent load, (W) = mass flow rate x latent heat of vaporization x (humidity difference) = 3010 x l/s x (∆ẁ) or (4840 x cfm x ∆ẁ) Where ∆ẁ = Inside-outside humidity ratio difference of air ( kg/kg)
  47. 47. Total Cooling Load • This is also call the Grand total load • Sum of – Space heat gain Room Total Load – System heat gain – load due to outdoor air supplied through the air handling unit • Air bypassed the coil • Air not bypassed the coil
  48. 48. System Heat Gain • These are sometimes external to the air conditioned space • HVAC equipment also contributes to heat gain – Fan heat gain – Duct heat gain
  49. 49. Bypass Factor Bypass factor is an important coil characteristic on moisture removal performance . It’s value depends on: • Number of rows/fins per inch • Velocity of air
  50. 50. Bypass Factor of the coil • When air streams across the cooling, portion of air may not come into contact with the coil surface • BPF = un-contacted air flow total flow BPF is normally selected at 0.1 for offshore cooling and dehumidification.
  51. 51. Typical Coil Bypass Factor Row Deep 14 fins/inch Face velocity= 2.5 m/s 3 m/s 2 m/s 1 0.52 0.56 0.59 2 0.274 0.31 0.35 4 0.076 0.10 0.12 6 0.022 0.03 0.04 Source: Refrigeration and Air Conditioning by CP Arora
  52. 52. Effect of Bypass Factor on Ventilation Load • Coil load due to outdoor air SH= (OASH)(1-BPF) LH= (OALH)(1-BPF) • Effective room load ERSH=RSH+(OASH)(BPF) ERLH=RLH + (OALH)(BPF)
  53. 53. Cooling Load Classroom Exercise • Estimate the cooling load N of a portal cabin shown here: • Assuming that – Outdoor condition is 35C, 70% RH 4x4 Platform – Indoor condition is 22C , x 3 h Lower Deck 55 % RH – U-factor=0.5 W/m2 K – Occupied by 2 persons – Electrical equipment heat is 3 kW – 100l/s leakage due to pressurization
  54. 54. Cooling Load Calculations Items Procedures Transmission- sensible Q = UA (CLTD) Wall- West side Wall- East side Wall – North Wall- South Roof Floor Total (T1) Internal load- sensible People Equipment Light Total (T2) Safety Factor (5% of T1+ T2) Fan heat & supply Duct Gain (7 % of T1+T2) RSH (Total of the above)
  55. 55. Coil Load Calculations Items Procedures Room Latent Heat (RLH) People Room Total Heat RSH + RLH
  56. 56. Cooling Load Calculations Items Procedures Design conditions Outdoor 35C, 70% RH Indoor 22C, 55 RH Ventilation- sensible Bypass air (0.1 bypass factor) 10% x outdoor air Sensible heat of bypass air Ventilation - Latent Latent heat of bypass air
  57. 57. Cooling Load Calculations Items Procedures Design conditions Outdoor 35C, 70% RH Indoor 22C, 55 RH ERSH RSH Sensible heat of air bypass Effective Room Sensible Heat ERLH People Latent heat of air bypass Effective Room Latent Heat Effective Room Total Heat (ERTH) ERSH+ESLH
  58. 58. Coil Load Calculation Items Procedures Design conditions Outdoor 35C, 70% RH Indoor 22C, 55 RH Coil Load – Sensible Effective Room Sensible Heat SH of Outdoor air not bypassed Total (Coil Sensible heat) Coil Load – Latent Effective Room Latent Heat LH of Outdoor air not bypassed Total (Coil latent heat) Total coil load (GTH)
  59. 59. Sensible Heat Factor (SHF) SHF RSHF ESHF GSHF
  60. 60. Sensible Heat Factor (SHF) • Ratio of sensible to total heat – SHF = Sensible heat/ total heat = SH/ (SH + LH) A low value of SHF indicates a high latent heat load, which is common in humid climate. • In the above example, – Calculate the SHF of the room (RSHF) – Calculate the effective room sensible heat factor (ESHF) – Calculate the SHF of the coil (GSHF)
  61. 61. Selection of Air Conditioning Apparatus • The necessary data required are: – GTH ( Grand total heat load) – Dehumidified air quantity – Apparatus dew point These determine the size of the apparatus and refrigerant temperature.

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