AHU air side cooling load calculation

by Dasa . 08 Dec 2020
Enter real values to get proper results:
Sensible Load in Tons:
Latent Load in Tons:
Total Load in Tons:
Sensible Heat Ratio:

Sensible cooling/heat load

The simple equation which we use for calculating sensible load is

Qs=1.08CFMΔTQ_{s} = 1.08 * CFM * \Delta T

The above equation is derived from

Qs=mCpΔTQ_{s} = m * C_{p} * \Delta T

Where,
QsQ_{s} - Sensible load
m - Mass flow rate of air
CpC_{p} - Specific heat of air
ΔT\Delta T - Temperature difference

To get the sensible load in Btu/hr by entering the air flow rate in cfm (cubic feet per minute), the other variables should have the below specified units.

mass flow rate in lb/ft3-hr,
Specific heat in Btu/lb-°F,
Temperature difference in °F

  • Mass flow rate of air = mass/volume
  • Density of air = 0.075 lb/ft3lb/ft^{3} at atmospheric pressure and 70oF70^{o} F
  • 1 cfm = 60 ft3/hourft^3/hour
  • Specific heat of air = 0.24 Btu/lboFBtu/lb-^{o}F

Substitute the above values and we get

Qs=(0.075cfm60)0.24ΔT=1.08cfmΔT\begin{aligned} Q_{s} &= (0.075 * cfm * 60) * 0.24 * \Delta T \\ &= 1.08 * cfm * \Delta T \end{aligned}

Latent cooling/heat load

The simple equation which we use for calculating latent load is

Ql=0.69CFMΔWQ_{l} = 0.69 * CFM * \Delta W

The above equation is derived from

Ql=mLΔWQ_{l} = m * L * \Delta W

Where,
QlQ_{l} - Latent load
m - Mass flow rate of air
L - Latent heat of water
ΔW\Delta W - Difference in humidity ratio

To get the sensible load in Btu/hr by entering the air flow rate in cfm (cubic feet per minute), the other variables should have the below specified units.

mass flow rate in lb/ft3-hr,
Latent heat of water in Btu/lb

  • Latent heat of water vapour = 1094 Btu/lb at atmospheric pressure and 75oF75^{o} F
  • Latent heat of condensed water = 18 Btu/lb at atmospheric pressure and 75oF75^{o} F
  • Latent heat of vapourizing water = 1094-18 = 1076 Btu/lb at atmospheric pressure and 75oF75^{o} F
  • Density of air = 0.075 lb/ft3lb/ft^{3} at atmospheric pressure and 70oF70^{o} F
  • 1 cfm = 60 ft3/hourft^3/hour
  • Specific heat of air = 0.24 Btu/lboFBtu/lb-^{o}F

Substitute the above values and we get

Ql=(0.075cfm60)1076ΔW=4842cfmΔW\begin{aligned} Q_{l} &= (0.075 * cfm * 60) * 1076 * \Delta W \\ &= 4842 * cfm * \Delta W \end{aligned}

Where,

ΔW\Delta W is in lb/lb

Ql=4842cfmΔW/7000=0.69cfmΔW\begin{aligned} Q_{l} &= 4842 * cfm * \Delta W / 7000 \\ &= 0.69 * cfm * \Delta W \end{aligned}

Where,

ΔW\Delta W is in gr/lb

Total cooling/heat load

Total load can be calculated by adding above two values viz., sensible load and latent load.

Qt=Ql+QsQ_{t} = Q_{l} + Q_{s}

OR we can calculate using below equation

Qt=mΔHQ_{t} = m * \Delta H

Where,
QtQ_{t} - Total load
m - Mass flow rate of air
ΔH\Delta H - Difference in Specific Enthalpy

Qt=0.075cfm60ΔH=4.5cfmΔH\begin{aligned} Q_{t} &= 0.075 * cfm * 60 * \Delta H \\ &= 4.5 * cfm * \Delta H \end{aligned}

Where,
QtQ_{t} - Total load in Btu/hr
ΔH\Delta H - Difference in Specific Enthalpy in Btu/lb

Sensible Heat Factor (SHF)

Sensible heat factor can be calculated using below equation

SHF=QsQs+QlSHF = \dfrac {Q_{s}} {Q_{s} + Q_{l}}


Don't be in delusion, understand the reality from historical data | Copyright © 2020 theDataTalks