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Cooling Load
Contents
•   Principle of cooling load
•   Why cooling load & heat gains are different
•   Design conditions
•   Understand CLTD/CLF method
•   An example
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
Purpose of Load Estimate
•   Load profile over a day
•   Peak load (basis for equipment sizing)
•   Operation Energy analysis
•   HVAC Construction cost
Principles of cooling Load Estimate
• Enclosure heat transfer characteristics
  – Conduction
  – Convection
  – radiation
• Design conditions
  – Outdoor & indoor
• Heat Gains
  – Internal
  – External or Solar
• Thermal capacity
Space Characteristics
•   orientation
•   Size and shape
•   Construction material
•   Windows, doors, openings
•   Surrounding conditions
•   Ceiling
Space Characteristics
•   Occupants (activity, number, duration)
•   Appliances (power, usage)
•   Air leakage (infiltration or exfiltration)
•   Lighting (W/m2)
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
Indoor Design Conditions
• Indoor air quality
  – Air contaminants
  – Air cleaning
• Acoustic requirements
• Pressurization requirements
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
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
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
Terminology
• Space- a volume without partition or a group
 of rooms
• Room- an enclosed space
• Zone- a space having similar operating
 characteristics
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
Heat Gain
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
Heat Gain = Cooling Load
Heat Gain = Cooling Load
Thermal storage and Construction Type
Time of the Day: Solar Radiation
Time-delay Effect: Lighting
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.
Heat Balance
          The principal terms of heat Gains/Losses are indicated below .




(Source: ASHRAE Handbook Fundamentals 2005)
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)
External Loads
1. Heat gains from Walls and roofs
  – sensible
2. Solar gains through fenestrations
  – Sensible
3. Outdoor air
  – Sensible & latent
Internal Loads
1. People
  – Sensible & latent
2. Lights
  – sensible
3. Appliances
  – Sensible & latent
Total Cooling Load
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
Refrigeration Load
• The capacity of the refrigeration system to
  produce the required coil load.
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
Heat Load Components


    Outdoor air &
    Electrical Equipment loads
    (77-85% )
                                           People: 3%

                                        Lighting: 4-8%

                                 Solar Transmission: 3-7%
                          Infiltration : 5-8%
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
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
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)
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
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
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
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
CLTD for Sunlit Wall Group G




Source: ASHRAE Fundamental
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
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
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
Internal Loads- People
• Q people-s = No x sensible heat gain/p x CLF
• Q people-L = No x latent heat gain/p
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
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
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
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)
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
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
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
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.
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
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)
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
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)
Coil Load Calculations
Items                    Procedures
Room Latent Heat (RLH)
People


Room Total Heat
RSH + RLH
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
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
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)
Sensible Heat Factor (SHF)
                  SHF

RSHF

ESHF

GSHF
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)
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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12 Cooling Load Calculations

  • 2. Contents • Principle of cooling load • Why cooling load & heat gains are different • Design conditions • Understand CLTD/CLF method • An example
  • 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. Purpose of Load Estimate • Load profile over a day • Peak load (basis for equipment sizing) • Operation Energy analysis • HVAC Construction cost
  • 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. Space Characteristics • orientation • Size and shape • Construction material • Windows, doors, openings • Surrounding conditions • Ceiling
  • 7. Space Characteristics • Occupants (activity, number, duration) • Appliances (power, usage) • Air leakage (infiltration or exfiltration) • Lighting (W/m2)
  • 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. Indoor Design Conditions • Indoor air quality – Air contaminants – Air cleaning • Acoustic requirements • Pressurization requirements
  • 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. 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. 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. Terminology • Space- a volume without partition or a group of rooms • Room- an enclosed space • Zone- a space having similar operating characteristics
  • 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
  • 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. Heat Gain = Cooling Load
  • 18. Heat Gain = Cooling Load Thermal storage and Construction Type
  • 19. Time of the Day: Solar Radiation
  • 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. Heat Balance The principal terms of heat Gains/Losses are indicated below . (Source: ASHRAE Handbook Fundamentals 2005)
  • 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. External Loads 1. Heat gains from Walls and roofs – sensible 2. Solar gains through fenestrations – Sensible 3. Outdoor air – Sensible & latent
  • 25. Internal Loads 1. People – Sensible & latent 2. Lights – sensible 3. Appliances – Sensible & latent
  • 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. Refrigeration Load • The capacity of the refrigeration system to produce the required coil load.
  • 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. Heat Load Components Outdoor air & Electrical Equipment loads (77-85% ) People: 3% Lighting: 4-8% Solar Transmission: 3-7% Infiltration : 5-8%
  • 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. 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. 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. 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. 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. 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. 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. CLTD for Sunlit Wall Group G Source: ASHRAE Fundamental
  • 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. 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. 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. Internal Loads- People • Q people-s = No x sensible heat gain/p x CLF • Q people-L = No x latent heat gain/p
  • 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. Coil Load Calculations Items Procedures Room Latent Heat (RLH) People Room Total Heat RSH + RLH
  • 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. 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. 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. Sensible Heat Factor (SHF) SHF RSHF ESHF GSHF
  • 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. 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.