9.1 Condenser Types and Operation

Heat Transfer Dynamics in Ammonia Condensation

The condenser is the primary high-side heat exchanger in the industrial vapor-compression cycle. High-pressure, high-temperature superheated ammonia (R-717) vapor is discharged from the compressor and directed to the condenser inlets. The fundamental objective is to reject the heat absorbed in the low-side evaporators, plus the heat of compression added by the compressors, to the surrounding atmosphere. This heat rejection changes the phase of the refrigerant from a vapor to a liquid.

Within the condenser, heat transfer occurs in three distinct thermodynamic zones:

1. Desuperheating Zone: The superheated discharge vapor enters the condenser coils (typically at temperatures between 150°F and 240°F, depending on compressor efficiency and pressure ratios). The gas is cooled sensibly until it reaches its saturation temperature. Since sensible heat transfer rates for gas are relatively low, this zone requires high-velocity flow to maximize the convection heat transfer coefficient.
2. Condensing Zone: Once cooled to its saturation temperature, the ammonia begins to condense. Latent heat is rejected at a constant temperature and pressure. The phase changes from vapor to liquid. R-717 has a remarkably high latent heat of vaporization (approximately 490 Btu/lb at a standard condensing temperature of 95°F), which means a small mass flow rate of ammonia can transfer a large amount of heat. This phase change occurs along the inner walls of the tubes, where liquid film forms and drains by gravity toward the outlet.
3. Subcooling Zone: Before the liquid leaves the condenser, it is cooled slightly below its saturation temperature. Subcooling ensures that the liquid remains in a stable state. If liquid is saturated at the condenser outlet, any friction or pressure drop in the piping will cause the liquid to immediately flash into vapor (forming flash gas). Flash gas reduces the capacity of expansion devices, starves the evaporators, and causes erratic system operation. A typical condenser is designed to provide 2°F to 5°F of subcooling.

Evaporative Condensers: Mechanical Construction and Operation

Evaporative condensers are the most common type used in large-scale industrial ammonia refrigeration. They combine the advantages of air-cooled and water-cooled systems, rejecting heat through the evaporation of water.

Air and Water Flow Configurations

An evaporative condenser consists of a galvanized steel coil pack, a water circulation pump, a water spray manifold, and fans. Saturated or superheated ammonia vapor flows inside the steel tubes. Water is pumped from a basin sump at the bottom of the unit to a spray header located above the coils. Low-clog spray nozzles distribute water downward over the tubes, forming a continuous thin liquid film. Simultaneously, fans move air through the coil pack. This flow can be configured in two ways:

• Induced Draft: The fan assembly is located at the top of the condenser, pulling air upward through the coils. This design yields highly uniform air velocity across the coil surface and reduces the risk of warm, humid exhaust air being drawn back into the air inlet (recirculation).
• Forced Draft: The fan assembly is located at the base of the unit, blowing air upward through the coils. This places the fan motors and belts in dry, ambient air, reducing corrosion risks and making mechanical components easier to access for routine maintenance.

Heat Transfer Mechanism

The primary mechanism of heat rejection is the latent heat of vaporization of water. As air moves counter-currently to the falling spray water, it causes a small fraction of the water film to evaporate. The heat required for this evaporation (approximately 970 Btu per pound of water evaporated at atmospheric pressure) is drawn directly from the steel tubes, cooling and condensing the ammonia inside. This is highly superior to sensible cooling (which relies on the specific heat of dry air, which is only 0.24 Btu/lb·°F).

Drift Eliminators

At the top of the condenser, the humid air stream passes through drift eliminators. These are a series of closely spaced, nested baffles. When the air stream flows through the zig-zag pathways of the eliminators, entrained water droplets collide with the plastic or metal surfaces and drain back into the sump. Drift eliminators are critical for:

• Water Conservation: Retaining water within the system.
• Environmental Safety: Preventing chemical-laden spray water from drifting onto surrounding structures.
• Pathogen Control: Minimizing the spread of aerosolized water droplets that could carry Legionella bacteria. Modern designs restrict drift loss to less than 0.005% of the total circulating water rate.

Sump Chemistry, Scaling, and Water Treatment

Because water is constantly evaporating, the minerals dissolved in the incoming makeup water (calcium, magnesium, silica) are left behind, gradually concentrating in the sump. This accumulation of minerals is measured as Total Dissolved Solids (TDS).

The Hazard of Scaling

If mineral concentrations exceed their solubility limit, they precipitate out and form a hard scale (calcium carbonate) on the exterior of the hot condenser tubes. Scale acts as a severe thermal insulator.

• The thermal conductivity of scale is extremely low compared to galvanized steel. Even a very thin layer of scale (e.g., 1/32 of an inch) dramatically reduces the heat transfer coefficient.
• As heat transfer degrades, the ammonia cannot reject heat effectively, raising the condensing temperature and head pressure.
• Efficiency Impact: For every 10 psi increase in head pressure, the compressor must work harder to achieve the higher pressure ratio, increasing compressor motor power draw by approximately 1.5% to 2%, while decreasing overall cooling capacity.

Water Management and Blowdown

To prevent scaling, operators must maintain water chemistry within strict parameters. This is achieved through:

1. Bleed-off (Blowdown): A controlled portion of the mineral-rich sump water is continuously or periodically drained (bled off) and replaced with fresh, low-mineral makeup water. This maintains the "cycles of concentration" (ratio of sump TDS to makeup water TDS) between 3 and 5.
2. Chemical Inhibitors: Anti-scaling chemicals are added to keep minerals in solution, even at higher concentrations.
3. Corrosion Control: The pH of the water must be monitored. For galvanized steel coils, the pH should be kept between 7.5 and 8.5. If the pH drops below 7.0, the water is acidic and will strip the zinc coating (white rust). If it rises above 9.0, it can cause rapid alkaline corrosion.
4. Biocides: Non-oxidizing and oxidizing biocides (chlorine, bromine) are added to control algae, slime, and bacteria. Biological slime acts as an insulator similar to scale and can promote under-deposit corrosion.

Air-Cooled and Water-Cooled Condensers

While evaporative condensers are standard, other technologies exist:

• Air-Cooled Condensers: Ammonia flows through heavily finned steel tubes while fans blow air across the exterior. Heat transfer is purely sensible. Since there is no water evaporation, the condensing temperature is limited by the air dry-bulb temperature. In hot weather, dry-bulb temperatures are high, leading to high approach temperatures (20°F to 30°F) and elevated head pressures. Consequently, they are less efficient, have a larger physical footprint, and require more fan power, but are used where water supply is scarce or water treatment is unavailable.
• Water-Cooled Condensers: These use a closed-loop water circuit, where ammonia is condensed in a heat exchanger using cooling water from an external source (like a cooling tower or river).
  • Shell-and-Tube: Features a large cylindrical shell containing multiple tubes. Ammonia vapor enters the shell and condenses on the outer surfaces of the tubes, while cooling water flows inside the tubes. Note that copper tubes are strictly prohibited in ammonia refrigeration due to chemical reactivity; steel or titanium tubes must be used.
  • Plate-and-Frame: Comprises stacked, corrugated metal plates. Ammonia and cooling water flow in alternating channels between the plates. This design provides high turbulence, leading to superior heat transfer rates, a low approach temperature, and a compact design.

Approach Temperature: The Operator's Diagnostic Tool

The approach temperature is defined as the difference between the refrigerant's condensing temperature (saturation temperature corresponding to discharge pressure) and the entering temperature of the cooling medium.

• Evaporative Condenser Approach: Condensing Temperature minus Entering Air Wet-Bulb Temperature. Wet-bulb temperature represents the absolute thermodynamic limit of evaporative cooling (the temperature air reaches when saturated with moisture). Under normal, clean operating conditions, the approach for an evaporative condenser should be 8°F to 15°F.
• Diagnostic Scenario: An operator notes that the ambient wet-bulb temperature is 78°F. The compressor discharge pressure gauge reads 166 psig. Referencing the saturated ammonia pressure table, a pressure of 166 psig corresponds to a condensing temperature of 90°F. Calculate the approach: 90°F - 78°F = 12°F. This indicates the condenser is operating within its normal design range. If the discharge pressure rises to 197 psig, the condensing temperature becomes 100°F. The approach increases to 100°F - 78°F = 22°F. This abnormally high approach indicates a failure: scaled or fouled coils, blocked nozzles, low water flow, fan failure, or air (non-condensables) trapped in the coils.

Condenser Type Performance Comparison