7.5 Evaporative Cooling Systems (Direct & Indirect Swamp Coolers): Principles & Maintenance

7.5 Evaporative Cooling Systems (Direct & Indirect Swamp Coolers): Principles & Maintenance

Evaporative cooling—commonly referred to in the American Southwest as "swamp cooling"—is one of the oldest and most energy-efficient methods of environmental climate control. In Arizona's arid Sonoran Desert, where summer outdoor wet-bulb temperatures frequently hover between 62°F and 70°F while dry-bulb temperatures reach 105°F to 115°F, evaporative cooling provides an exceptionally low-cost cooling alternative to vapor-compression refrigeration.

Evaporative cooling consumes up to 70% to 80% less electrical energy than traditional direct-expansion (DX) air conditioning because electrical power is required only to operate a low-pressure water pump and a fractional-horsepower blower motor, rather than a heavy refrigeration compressor.

1. Direct Evaporative Cooling (DEC) Thermodynamics

Direct Evaporative Cooling operates on the physical principle of adiabatic saturation. When warm, dry outdoor air is drawn through water-saturated media pads, water evaporates into the air stream.

                      DIRECT EVAPORATIVE COOLING THERMODYNAMICS

        Entering Outdoor Air                         Leaving Supply Air
        (Hot & Arid)                                 (Cooled & Humidified)
        ┌──────────────────┐    ┌──────────────┐    ┌──────────────────┐
        │ T_db = 105°F     │───►│ WETTED MEDIA │───►│ T_db = 74°F      │
        │ T_wb = 66°F      │    │  SATURATION  │    │ T_wb = 66°F      │
        │ W = 40 gr/lb     │    │   PAD (80%)  │    │ W = 85 gr/lb     │
        │ Enthalpy = 30.8  │    └──────────────┘    │ Enthalpy = 30.8  │
        └──────────────────┘                        └──────────────────┘
        ◄──────────────── CONSTANT ENTHALPY (h) & WET-BULB ──────────────►

Thermodynamic Laws of DEC:

1. Sensible-to-Latent Energy Exchange: Sensible heat from the dry air provides the latent heat of vaporization (1,061 BTU per pound of water evaporated). The sensible heat reduction drops the air dry-bulb temperature.
2. Constant Enthalpy & Wet-Bulb: Because no external heat is added or subtracted (Q_total = 0), the thermodynamic process follows a line of constant enthalpy (h) and constant wet-bulb temperature (T_WB) on the psychrometric chart.
3. Theoretical Lower Limit: The lowest theoretical temperature to which air can be cooled by direct evaporative cooling is the ambient wet-bulb temperature (T_WB).

2. Saturation Efficiency & Supply Air Temperature Calculations

No evaporative cooler achieves 100% theoretical saturation in practice. The degree of approach to the ambient wet-bulb temperature is quantified by the Saturation Efficiency (η) (also known as evaporative effectiveness):

η = (T_DB,entering - T_DB,supply) / (T_DB,entering - T_WB,entering)

Rearranging to calculate the delivered Supply Air Temperature (T_supply):

T_supply = T_DB,entering - [η × (T_DB,entering - T_WB,entering)]

Evaporative Media Comparison Table

Worked Engineering Examples

Example 1: Supply Temperature with Aspen Media
Scenario: Outdoor conditions in Tucson are 104.0°F DB and 66.0°F WB. An evaporative cooler with standard Aspen wood pads operates at an efficiency η = 75% (0.75). What is the supply air dry-bulb temperature?

1. Calculate wet-bulb depression:
   Depression = 104.0°F - 66.0°F = 38.0°F
2. Apply the saturation formula:
   T_supply = 104.0°F - (0.75 × 38.0°F) = 104.0°F - 28.5°F = 75.5°F

Example 2: Supply Temperature with 12" Rigid Cellulose Media
Scenario: Using the identical outdoor conditions (104.0°F DB / 66.0°F WB), a commercial unit is upgraded to a 12-inch rigid cellulose media pad with an efficiency η = 90% (0.90). What is the resulting supply temperature?

1. Apply the formula:
   T_supply = 104.0°F - (0.90 × 38.0°F) = 104.0°F - 34.2°F = 69.8°F

Thermodynamic Insight: The high-efficiency rigid media delivers supply air that is 5.7°F colder, breaking below the 70°F threshold during peak 104°F desert heat.

3. Airflow Sizing: The Air Change Method

Unlike direct-expansion (DX) systems that recirculate indoor air (80% to 90% Return Air), direct evaporative coolers introduce 100% Outdoor Air into the building. To maintain continuous airflow and purge humidity, the building must have dedicated positive exhaust relief pathways (such as opened windows or ceiling Up-Ducts into the attic).

Because supply air temperatures from evaporative coolers (70°F to 76°F) are warmer than mechanical DX refrigeration supply air (52°F to 56°F), evaporative cooling requires 3 to 4 times more volumetric airflow (CFM) to absorb the space heat gain.

The Air Change Sizing Formula:

CFM_evaporative = Conditioned Building Volume (cu ft) / Air Change Rate (Minutes per Air Change)

Conditioned Volume = Floor Area (sq ft) × Ceiling Height (ft)

Climate Air Change Rate Standards:

• Extreme Arid Desert (Phoenix, Yuma, Lake Havasu): 1.0 to 1.2 minutes per air change (50 to 60 air changes per hour).
• Moderate Arid (Tucson, Prescott): 1.3 to 1.5 minutes per air change (40 to 46 air changes per hour).

Worked Example: Sizing a Phoenix Home

Problem: A single-story residence in Phoenix has 2,200 sq ft of conditioned floor space with 9.0 ft ceilings. What is the required evaporative cooler airflow rating assuming a 1.2-minute air change design standard?

1. Calculate total interior volume:
   Volume = 2,200 sq ft × 9.0 ft = 19,800 cu ft
2. Apply the Air Change Sizing formula:
   CFM = 19,800 cu ft / 1.2 minutes = 16,500 CFM

Contractor Equipment Selection: The contractor installs two 8,500 CFM units (or one 16,500 CFM rooftop package). By contrast, a DX air conditioner for the same home would require only ≈ 1,600 to 1,800 CFM (4 to 4.5 tons).

Relief Exhaust Area Sizing Rule:

To prevent backpressure and blower stalling, the building must provide at least 1.0 sq ft of open relief area per 300 CFM of cooler airflow (or 2.0 sq ft if insect screens are present). For a 16,500 CFM system:
Net Relief Area = 16,500 CFM / 300 CFM/sq ft = 55.0 sq ft of net open window/damper area.

4. Indirect Evaporative Cooling (IEC) and Two-Stage (IDEC) Systems

1. Indirect Evaporative Cooling (IEC)

In an Indirect Evaporative Cooler, evaporation occurs in a separate, isolated secondary air stream. The secondary air and water spray cool the exterior surfaces of an air-to-air plate heat exchanger or heat pipe bundle. The primary indoor supply air passes through dry internal channels, giving up sensible heat through the heat exchanger plates.

• Key Advantage: Achieves substantial sensible dry-bulb temperature reduction with ZERO increase in indoor moisture content (W remains completely constant).
• Limitation: Lower saturation efficiency than DEC (η ≈ 55% to 65%).

2. Two-Stage Indirect/Direct Evaporative Cooling (IDEC)

Two-stage systems combine indirect and direct evaporative processes in series:

• Stage 1 (Indirect Pre-Cooling): Outside air is first sensibly cooled across the indirect heat exchanger without adding moisture. This drops both the dry-bulb and wet-bulb temperatures (e.g., 105°F DB / 66°F WB is cooled to 80°F DB / 58°F WB).
• Stage 2 (Direct Adiabatic Saturation): The pre-cooled air enters a high-efficiency direct rigid media pad. Because the entering wet-bulb was depressed in Stage 1, Stage 2 produces supply air at 60°F to 64°F—comparable to mechanical DX air conditioning while using 60%+ less electricity!

5. Water Quality Management, TDS & Maintenance

In Arizona, municipal water supplies sourced from the Colorado River and groundwater aquifers contain high concentrations of dissolved calcium and magnesium carbonates (water hardness: 200 to 450+ PPM).

                      MINERAL ACCUMULATION IN EVAPORATIVE SUMP

            Water Evaporates (Pure H2O Vapor) ──► Leaves Minerals in Sump
                                                     │
                                                     ▼
            Total Dissolved Solids (TDS) Surges ──► Hard Calcium Carbonate Scale
                                                     │
             ┌───────────────────────────────────────┴──────────────────────┐
             ▼                                                              ▼
   UNCONTROLLED SUMP                                      CONTROLLED SUMP
   • Media scales solid within 60 days                    • Continuous Bleed-off Line (1-3 GPH)
   • Airflow drops 50%+                                   • Or Timed Dump Pump (Every 2-4 hrs)
   • Motor burns out from overload                        • Media remains clean for 5+ years

Continuous Bleed-Off vs. Automatic Sump Dump

As pure water vapor evaporates from the media pads, mineral salts are left behind in the sump water. Without active mitigation, the Total Dissolved Solids (TDS) concentration surges, causing calcium carbonate (caliche) scale to encrust the media pads, severely restricting airflow and harboring bacteria.

1. Continuous Bleed-Off Line: A small metering bleed line installed on the pump discharge tube diverts a continuous stream of mineral-rich water (1.0 to 3.0 Gallons per Hour per 1,000 CFM) to an exterior drain, continuously replenishing the sump with fresh makeup water.
2. Automated Timed Sump Dump: An electronic timer energizes an electric drain pump every 2 to 4 hours of run time, dumping the entire mineral-concentrated sump volume (5 to 10 gallons) and refilling it with fresh water. This saves water compared to continuous bleed-off while completely eliminating scale formation.

Seasonal Maintenance Protocols

• Spring Startup: Inspect blower bearings and lubricate with SAE 20 non-detergent oil; verify belt tension (1/2 to 3/4-inch deflection at mid-span); clean sump pan; install new media pads; set float valve level (1 inch below overflow pipe); verify uniform water distribution across all pad troughs.
• Fall/Winter Shutdown: Disconnect electrical power; shut off and drain water supply line (blow out with compressed air to prevent freezing); drain and sanitize sump pan; remove water pump and store dry; install an exterior canvas or sheet metal winter weather damper cover over the unit cabinet to prevent cold winter drafts and conditioned air escape.