# Component characteristics Fan Heat Gain

Due to irreversibility, i. e. friction, fan contributes to sensible heat gain, and increases temperature of moist air.

Where — W = PFm, therefore Tout = Tin +

Macv

Example

A 1.5 kW fan moves lm3/s of dry air entering at 15°C. What is Tout? Solution:

Duct and Plenum Heat Gain

Plenum heat gains: lights; hot pipes.

Ducted supply/return: convection in hot spaces; “sweating” on cool duct surfaces.

Example

 Mixing Section

 ;o — 90 F 0O = 55% — Q0 = 500 cfm

 Q = 60,000 Btu/hr qs = 42,000 Btu/hr

 Cooling and Dehumidifying Unit

 Supply fan

Cooling Coils

Cooling coil are indirect contact heat exchangers. Different types:

• Air-to-Water

• Air-to-Refrigerants

Air is outside, and liquid is inside the tubes.

Air-side — fins promote heat transfer (larger area)

Water-side — shape promotes heat transfer (higher turbulence)

For an ideal coil: TDb = Tcou, mean

0= 100%

For a real coil: TDb > Tcol|, mean

0< 100%

 Cooling

 Condensate Drain

 Condensate [

 10-3

Approximate methods for rough estimate of cooling coil condition:

• fixed relative humidity (assume coil leaving RH, say 90%)

• bypass factor “b” (assume fraction of flow bypasses the coil, reminder in perfect contact with coil)

O

Where LA-air leaving the coil, EA — air entering coil, ADP — coil (apparatus) dew point Example:

A chilled water coil with 8°C entering water conditions air from 26°C db/ 19°C wb to 15°C db/ 14°C wb. What are Tadp, b, and leaving relative humidity?

Solution:

Evaporative Cooling

If sufficiently dry air is available, an evaporative process can be used to cool the air stream. Direct evaporative coolers (see Figure) add moisture to air adiabatically. The evaporation uses air sensible heat => Tair drops.

Direct evaporative cooling effectives:

Where Јe varies with air flow rate and media thickness. Range is 60-95%. Typical value is 80%. Unassisted Direct Evaporative Cooling

Applicable if the wet bulb temperature is less than 24°C (75°F) Regional applicability is limited in U. S.:

• the western U. S.

• the north central states

• the northeastern U. S.

May require large supply airflow rates. May give high space humidity at times.

 Indirect evaporative cooling is sensible process of cooling an outdoor air stream without humidifying (sensible cooling). Indirect evaporative cooling effectives: TA° F)

Range is 40-80%.

Benefits of evaporative cooling:

Large savings possible, “environmentally friendly”, inexpensive for direct cooling.

Problems of evaporative cooling:

May not meat peak loads, may increase duct size, may allow wider variation of humidity, potential microbial growth problems.

 Coil evap. coil evap. Cooling coil cooler

 Heating and Humidification Systems

Example of a system that extends application of direct and indirect coolers. The system better control humidity by combining evaporative and mechanical cooling, and increase energy efficiency by including an economizer.

Humidification types: wetted media, heated pan, atomizer and steam.

 MOIST A»» Jacketed Steam Humidifier Wetted Drum Humidifier

 DUCT

 WATER VAPOR

 OVERFLOW DRAIN

 SUPPLY WATER

 HEAT EXCHANGER ELECTRIC, STEAM, OR HOT WATER

 TO DRAIN PLUG FOR PAN BLOWDOWN

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-ELIMINATORS

Atomizing Humidifier with Optional Filter Eliminator

Heated Pan Humidifier