warehouse air conditioning

How Hybrid Indirect Evaporative Cooling Systems Yield 80% Savings Over Standard Air Conditioning

ENGINEERING SPECIFICATIONS

DEFINITIONS

  • DRY BULB TEMPERATURE is the temperature determined by an ordinary thermometer.

  • WET-BULB TEMPERATURE is the temperature of an air sample passed over a large surface of liquid water in an insulated channel.  Wet-bulb and dry-bulb equal each other when the air sample is saturated.

  • DEW POINT TEMPERATURE is the temperature of a moist air sample at the same pressure required to reach water vapor saturation.  At this point water vapor condenses into a liquid.

  • RELATIVE HUMIDITY is the ratio of water vapor in an air sample compared to water vapor in a saturated sample at the same temperature and pressure.  Units are expressed as a percent.

HOW EVAPORATIVE COOLING WORKS

In a given ambient air sample, dry bulb is higher than wet bulb and wet bulb is higher than dew point.  For example, to air condition a warehouse in Corona, California the conditions are:

  • 105 F Dry bulb

  • 75 F Wet bulb

  • 65 F Dew point.

Traditional Air Conditioning

Traditional Air Conditioning is a refrigeration cycle where refrigerant boils at 38 to 42 degrees F. This lowers the leaving air temperature to 48 to 52 degrees F. This leaving air temperature is below dew point and condenses out water from the air being cooled.

When indoor temperature is 74 F, the resulting relative humidity is 45%. Most people consider this relative humidity comfortable, however some industrial applications require higher relative humidity.  A humidifier is required for additional moisture. Energy is required for humidification. This wastes energy.

Indirect Cooling - Engineering Schematic

DIRECT EVAPORATIVE COOLING

DIRECT EVAPORATIVE COOLING involves adding water to the air.  It requires a continuous supply of outdoor air to work. Applications require substantial exhaust and areas of low relative humidity. This method is acceptable provided there is low internal heat load. Leaving air is about 5 to10 F higher than outdoor wet bulb and is high in relative humidity.  Substantial fan energy is required, however it operates on only 20% of the energy of traditional air conditioning.

CONCLUSION

Direct evaporative cooling is effective but has these drawbacks:

1. It requires a dry climate
2. It requires buildings with low internal heat gain
3. It results in humid indoor air
4. It is relatively limited in cooling power.

 

INDIRECT EVAPORATIVE COOLING

INDIRECT COOLING BY EVAPORATION (IDC) produces cool air indirectly by using a cooling tower to produce cool water (about 3F higher than wet bulb). Then it uses a heat exchanger to cool circulating air.  Leaving air temperature is 7 F higher than wet bulb.  This method competes with direct cooling dry bulb indoor temperatures but does not add water to the air.  Indoor relative humidity is slightly higher than outdoor relative humidity because cooler air holds less water vapor.

TWO STAGE IDC

TWO STAGE IDC uses the cooling tower to reduce the dry bulb and wet bulb temperature of air entering the cooling tower. The result is water temperature at 3 to 6 degrees F lower than outdoor wet bulb temperature. Leaving air temperature is usually about 3 degrees F lower than outdoor wet bulb temperature.  In most weather conditions, leaving air temperature creates satisfactory room temperatures that compete with traditional air conditioning but with 85% less energy.

Note: in very humid climates, IDC may not be adequate.

MODULATING PUMPS AND FANS

Variable frequency drives (VFDs) added to IDC decrease electrical consumption and increases power under less than full load conditions.  Slowing the outdoor pumps allows the towers to achieve a lower temperature (water temperature minus wet bulb temperature).  VFDs are throttled  by modulating thermostats and Programmable Logic Controllers (PLCs).  Modulating thermostats control circulating fan speed while PLCs modulate the speed of the outdoor cooling tower fans and pumps. The net result is additional cooling for less energy under turned down conditions.

ELECTRIC COMPANY REBATES

Since IDC is cheaper to operate than traditional air conditioning electric companies typically rebate one year of saved energy.  This rebate is usually about 30% of the cost of the installation.

ROOM PRESSURIZATION

For buildings requiring a high level of cleanliness, air filtration and positive room pressure are effective.  We accomplish positive room pressure with low differential pressure sensors and VFDs with PID (proportional integral derivative) control.

CONCLUSIONS

1. Different forms of cooling can be combined (D-X, DIRECT EVAPORATIVE, IDC, 2 STAGE IDC) in hybrid form for the most energy efficient cooling under all climate situations.

2, Applications involving high amounts of air infiltration are energy intensive and benefit substantially from these hybrid systems.

3. Electric utility companies often pay for up to 30% of the installed project.

4. Compared to traditional air conditioning the net result is an 80 percent reduction in electrical energy.

5. IDC maintains a comfortable environment for workers and improves industrial processes.