Click here for Air Handling Unit (AHU) Component Selection Guide.

BTU of the coil side(High side) should match the BTU of the air side(Low side).

Typical values are

- Chilled water-in 44° F
- Chilled water-out 54° F
- Estimated Δt - 10° F
- Chilled water pressure drops - max. 20 ft of H2O
- Flow rate - 2.4 gpm/ton
- Water velocity – 2 fps ~ 3 fps

A typical chilled water system gives water at 44° F to an AHU at the inlet.

Measure the water flow rate at the inlet of the AHU. We can measure the flow rate using an analog/digital manometer OR an ultrasonic flowmeter installed on the chilled water pipe.

Click here for Chilled water flows measurement using Orifice plate, Balancing valves, Manometers & Pitot tubes.

Typical AHU requires a flow rate of 2.4 gallons/ton. So, the flow rate should be 24 gpm for the AHU capacity of 10 TR.

A typical AHU will have a Δt of 10° F, hence we assumed the AHU coil outlet temperature as 54° F.

Refer water system cooling load calculations for formulae and derivations.

For the above assumed parameters we can calculate the cooling load as

```
Btu/hr = 500 x gpm x Δt
= 500 X 24 gpm x (44° F - 54° F)
= 120000
= 120000 / 12000 tons = 10 tons
```

For simplicity consider there is no fresh air addition, AHU fan motor heat gain, and infiltration.

Also a typical cooling and dehumidification process is considered in this example.

Use “sling psychrometer” and take the below readings at the AHU inlet.

- DBT = 75° F
- WBT = 68° F

AHU inlet is same as the “return air from room” / “room air temperature” / “required zone air conditions”.

Also, take the below readings at the AHU coil outlet.

- DBT = 57° F
- WBT = 56° F

Coil leaving air temperature will be slightly higher than the coil surface temperature & chilled water outlet temperature. Here the assumed chilled water outlet temperature is 54° F. Leaving air temperature is dependent on

- The surface area of the coil in contact with the air stream
- The velocity of the air stream

Higher the contact area and higher the time in contact with the surface gives less temperature difference and better efficiency.

At 100% efficiency, the coil leaving air dry bulb temperature (DBT) will be equal to wet bulb temperature (WBT) since the air is saturated.

We usually get a temperature difference of less than 1° F between DBT and WBT hence 100% efficiency is always not possible.

Resolve other psychrometric properties of air at inlet & outlet of the AHU coil.

Refer Psychrometric chart, properties calculator and related equations.

The below specified values in the table were taken from the psychrometric properties calculator & not the from the chart.

Property | Description | Values @ Inlet | Values @ Outlet |
---|---|---|---|

HR (OR) AH | Humidity Ratio (OR) Absolute Humidity | 0.013 lb/lb | 0.0093 lb/lb |

VP | Vapour Pressure | 0.302psia | 0.217 psia |

RH | Relative Humidity | 70.3% | 94.16% |

DP | Dew Point Temperature | 64.7° F | 55.4° F |

H | Enthalpy | 32.3 Btu/lb | 23.8 Btu/lb |

SV | Specific Volume | 13.76 ft3/lb | 13.22 ft3/lb |

Calculate the air flow rate in CFM (cubic feet per minute) passing through the AHU coil, using an Anemometer.

Assumed measured flow rate is given below.

```
Air flow rate in CFM = 3000
```

Total cooling load can be calculated using the above enthalpy values & air flow rate

```
Total cooling load Q<sub>total</sub> = 4.5 * CFM * ΔH
= 4.5 * 3000 * (32.3 - 23.8)
= 114,750 Btu/hr
= 9.56 Tons
```

Refer air side energy calculation for the equations and calculate the below values

```
Sensible cooling load Qs = 4.86 tons
Latent cooling load Ql = 4.48 tons
Total cooling load Qt = 9.34 tons
Sensible heat factor SHF = 0.52
```

Refer ADP and BF calculation for the equations and calculate the below values

```
Apparatus dew point temperature ADP = 53.7° F
By-pass factor = 0.15
```

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