SENSIBLE HEAT FACTOR (SHF)

CLASS NOTES FOR ENGINEERING

Sensible means due to temperature

difference. Consider both sensible and

latent heat loads in air conditioning.

 Remove these in the same ratio in which

added. Suitable sensible heat factor

makes air conditioning enjoyable.

There are many types of sensible heat factors.

(a) ROOM SENSIBLE HEAT FACTOR (RSHF)

RSHF=RSHL/(RSHL+RLHL) =RSHL/TRHL

where RSHL is room sensible heat load

RLHL is room latent heat load

 TRHL is total room heat load

Example – Room Sensible Heat Factor – RSHF

The sensible heat gain in a room is 40 kW while The latent heat gain is 16 kW.

The Room Sensible Heat Factor will be:

RSHF = (40 kW) / ((40 kW) + (16 kW)) = 0.71

(b) Effective room sensible heat factor = ERSHF                     

             =ERSHL/(ERSHL+ERLHL)

(i) Effective room sensible heat load = ERSHL

  Outside air sensible heat load = OASHL

ERSHL= RSHL+ BPF. OASHL  

Where   BPF = By pass factor     

Assume BPF=0.1or 0.15 if not given

(ii) Effective room Latent heat load = ERLHL

ERLHL=RLHL+BPF .OALHL

(iii) OASHL=Volumetric flow rate of ventilated air x density (t_dbo – t_dbi)

(iv) OATHL= Volumetric flow rate x density (h_ODC — h_IDC)

OALHL=OATHL—OASHL

OR

OALHL = Volumetric flow rate x density (w_ho –w_hi)x LH of water

(c) DIFFERENT CALCULATIONS REQUIRED 

(i) AIR VOLUME CALCULATION

Dehumidified supply air volume=V_s (m³/s)= ERSHL/(ρc_p(t_rdbt—t_ADP))

t_{rdbt  is the room dry bulb temperature} 

t_{ADP is the Apparatus Dew Point temperature}

(ii) Outside air flow rate= V_o (m³/s)=number of occupants x 0.10      m³/s

Where 0.10 m³/s  fresh air required per person per second (assumed)

(d) Grand sensible heat factor =GSHF 

          = Total sensible heat factor  

          = GSHL/(GSHL+GLHL)

GSHL=RSHL+OASHL

GLHL= RLHL+OALHL

GRAND and total are the same thing.

(e)  Apparatus Dew Point (ADP) is the Cooling coil (Evaporator) temperature. Determine it It can in the following ways:

(i) Apparatus Dew Point (ADP) is the point where ERSHF line meets the saturation curve. Draw ERSHF line from the room condition (IDC condition) parallel to the ESHF line

 (ii) Apparatus Dew Point (ADP) is the point where OAGSHF line meets the saturation curve. Draw OAGSHF line from the mixture condition parallel to the OAGSHF line

(iii)  Apparatus Dew Point (ADP) is the point where OASHF line meets the saturation curve. Draw OASHF line from the outside design condition (ODC condition) parallel to the OASHF line

There is a  SHF scale on the psychrometric chart. There is a standard point  27⁰C with 50 % RH. This standard point is often  reference point or reference circle or alignment circle.

Use a reference point for the sensible to latent heat ratio lines.  Taken out of heat from the room is in the same ratio (SH/LH) in which added. This makes room conditions comfortable.

Draw a  reference line joining the RSHF to the alignment circle. SHF LINE is a line joining the SHF on the scale and an alignment point on the chart.

 Room Apparatus Dew Point (RADP) is the point where RSHF line meets the saturation curve. Draw RSHF line from the room IDC condition parallel to the SHF line.