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BLD-HVAC07-07 · Lesson 5 · 12 clock hours · demo

Condensers and Condenser Controls in Commercial Applications

Objective. Students will describe air-cooled and evaporative condenser operation, measure condenser leaving air temperature and approach temperature, and explain head-pressure control strategies used in cold ambient conditions.

Condensers and Condenser Controls in Commercial Applications

The condenser is the component in a refrigeration system where heat absorbed from the refrigerated space is rejected to the environment. In commercial refrigeration, two condenser types dominate: air-cooled and evaporative. Understanding how each works—and how to control head pressure across a range of ambient conditions—is essential for safe, efficient service work.

Air-Cooled Condensers

An air-cooled condenser moves outdoor air across a coil of refrigerant-carrying tubes fitted with aluminum fins. The refrigerant enters the coil as a hot, high-pressure vapor from the compressor discharge line. As air flows across the fins, it absorbs heat from the refrigerant, causing the refrigerant to condense into a liquid. That liquid then flows to the receiver or metering device.

Capacity depends on two things: the temperature difference between the condensing refrigerant and the entering air, and the airflow rate across the coil. The wider the temperature difference, the greater the heat rejection rate. As ambient temperature rises on a hot summer day, the difference shrinks, head pressure climbs, and system efficiency falls. Keeping coils clean and fans running at full capacity is therefore critical during peak load.

Evaporative Condensers

An evaporative condenser replaces part of the sensible-heat transfer with evaporative cooling. Refrigerant coils are housed inside a cabinet; water is sprayed over the coil while a fan draws air through the unit. The evaporation of water dramatically increases heat rejection capacity compared with a dry air-cooled unit of similar physical size. Because performance is tied to the wet-bulb temperature of the entering air rather than the dry-bulb temperature, evaporative condensers work especially well in hot, dry climates. They require water treatment to prevent scale and biological growth, and winterization is necessary in freezing climates to drain water circuits.

Approach Temperature

Approach temperature is a key diagnostic measurement for any condenser:

> Approach Temperature = Condensing Temperature − Entering Air Temperature (dry-bulb)

The condensing temperature is found by converting the measured discharge (head) pressure to a saturation temperature using the pressure-temperature chart for the refrigerant in use. A higher-than-normal approach temperature indicates reduced condenser performance—typically caused by dirty coils, blocked airflow, failed fans, or non-condensable gases in the system.

Head-Pressure Control in Cold Ambient Conditions

Commercial refrigeration systems are often located outdoors and must operate year-round. On cold days, the ambient temperature can fall well below the design condition, causing condensing pressure to drop too low. A low head pressure reduces the pressure difference across the metering device, starving the evaporator of refrigerant and causing operational problems including compressor short-cycling and refrigerant flood-back.

Three common head-pressure control strategies are used:

1. Fan Cycling Control

A pressure-activated switch (head-pressure controller) cycles condenser fans off as head pressure drops, reducing airflow and allowing condensing pressure to recover. Simple and low-cost, but can allow pressure to hunt (cycle repeatedly) in variable ambient conditions.

2. Fan Speed Control (Variable Frequency Drive — VFD)

A VFD modulates the motor speed of one or more condenser fans in response to head pressure. Smoother control than fan cycling, better energy efficiency, and less mechanical stress on fan motors. As ambient temperature drops, the VFD slows the fan to maintain a stable condensing pressure setpoint.

3. Condenser Flooding (Refrigerant Flooding Valve)

Used on some receiver-equipped systems, a flooding valve backs liquid refrigerant up into the condenser coil, reducing the effective condensing surface and raising head pressure. Common on older commercial systems.

Safety Reminders

What to Expect in the Field Activities

During the field walk, you will use a digital thermometer to measure ambient (entering) air temperature and leaving air temperature at the condenser discharge, and a manifold gauge set to read discharge pressure. Convert discharge pressure to condensing temperature using the refrigerant's P-T chart, then subtract ambient temperature to find approach temperature. Record every reading on your data sheet—accurate field notes are a professional habit.

On the training rig, observe how adjusting the head-pressure controller setpoint changes the cycling behavior of condenser fans and affects the discharge pressure reading on the manifold gauge. Note amperage changes with a clamp ammeter to see the energy effect of fan staging.

In the case study exercise, apply what you have learned: a unit short-cycling in cold weather almost always points to a head-pressure control problem. Your job is to identify which control strategy is missing or misconfigured and justify your answer with data.

Key terms8

Approach TemperatureThe difference between the condensing (saturation) temperature at the measured head pressure and the dry-bulb temperature of the air entering the condenser; indicates condenser efficiency.
Head PressureThe high-side refrigerant pressure measured at the compressor discharge or condenser inlet; also called discharge pressure or condensing pressure.
Condensing TemperatureThe saturation temperature of the refrigerant at the measured head pressure, read from a pressure-temperature (P-T) chart for that refrigerant.
Fan Cycling ControlA head-pressure control method that uses a pressure switch to turn condenser fans on or off, maintaining minimum condensing pressure during cold ambient conditions.
Variable Frequency Drive (VFD)An electronic device that varies the speed of an AC motor by changing the frequency of the electrical supply; used on condenser fans to modulate airflow and maintain stable head pressure.
Wet-Bulb TemperatureThe temperature a thermometer reads when its sensing element is covered with a wetted wick; governs evaporative cooling performance and is always equal to or lower than dry-bulb temperature.
Condenser FloodingA head-pressure control strategy that uses a valve to back liquid refrigerant into the condenser coil, reducing active condensing surface area and raising head pressure in cold weather.
Non-Condensable GasesAir or other gases that do not condense at refrigeration operating temperatures and pressures; they raise head pressure and reduce condenser capacity.

Worked examples4

1. A technician is servicing a walk-in cooler condensing unit using R-404A. The measured discharge pressure is 200 psig. The ambient (entering) air temperature at the condenser is 75 °F. What is the approach temperature?
  1. Step 1 — Convert discharge pressure to condensing temperature using an R-404A P-T chart. At 200 psig, R-404A has a saturation temperature of approximately 100 °F.
  2. Step 2 — Apply the approach temperature formula: Approach Temp = Condensing Temp − Entering Air Temp = 100 °F − 75 °F = 25 °F.
  3. Step 3 — Evaluate the result. A clean, properly sized air-cooled condenser typically shows an approach temperature of roughly 10–20 °F under normal conditions. A value of 25 °F suggests possible dirty coils or restricted airflow worth investigating.
Answer: Approach temperature = 25 °F, indicating a potential performance issue worth further inspection.
2. On a cold winter day, the ambient temperature drops to 20 °F. The head-pressure controller is set to cycle the last condenser fan off at 140 psig (R-404A). What is the condensing temperature the system is trying to maintain at that setpoint?
  1. Step 1 — Look up R-404A saturation temperature at 140 psig on a P-T chart. At 140 psig, R-404A saturates at approximately 70 °F.
  2. Step 2 — Confirm the logic: by cycling the fan off at 140 psig, the controller is preventing condensing temperature from falling below about 70 °F, which keeps enough pressure differential across the metering device for proper refrigerant feed.
  3. Step 3 — Calculate the approach temperature if the system holds 140 psig with 20 °F ambient: 70 °F − 20 °F = 50 °F. This wide spread is expected because airflow is being restricted intentionally to hold head pressure up.
Answer: The system maintains a condensing temperature of approximately 70 °F at the 140 psig cutout setpoint; approach temperature in this condition is approximately 50 °F.
3. A condenser leaving-air temperature is measured at 118 °F, and the ambient entering-air temperature is 95 °F. What is the temperature rise across the condenser coil, and what does a large rise suggest?
  1. Step 1 — Calculate coil temperature rise: 118 °F − 95 °F = 23 °F rise across the condenser.
  2. Step 2 — Interpret the result. A larger temperature rise means the air is picking up more heat per unit of airflow. This can indicate reduced airflow (failed fan, blocked inlet or outlet) rather than a coil-fouling issue, which would show up more clearly in the approach temperature calculation.
  3. Step 3 — Cross-check with approach temperature: measure or calculate condensing temperature from head pressure and subtract ambient. If approach is also elevated, both restricted airflow and dirty coils are suspect.
Answer: Temperature rise = 23 °F. An unusually large rise relative to design suggests restricted airflow; compare with approach temperature to distinguish airflow from coil-fouling problems.
4. A technician replaces a failed condenser fan motor on a three-fan air-cooled condenser. Before the repair, discharge pressure on R-448A was 320 psig in 85 °F ambient. After restoration of all three fans, pressure drops to 270 psig. What are the condensing temperatures before and after the repair? (Use approximate saturation values: R-448A at 320 psig ≈ 115 °F; at 270 psig ≈ 100 °F.)
  1. Step 1 — Before repair: condensing temperature at 320 psig = 115 °F. Approach temperature = 115 °F − 85 °F = 30 °F.
  2. Step 2 — After repair: condensing temperature at 270 psig = 100 °F. Approach temperature = 100 °F − 85 °F = 15 °F.
  3. Step 3 — Compare: approach temperature dropped from 30 °F to 15 °F, confirming the failed fan was the cause of elevated head pressure and confirming the repair was successful.
Answer: Before: condensing temp ≈ 115 °F, approach ≈ 30 °F. After: condensing temp ≈ 100 °F, approach ≈ 15 °F. The 15 °F reduction in approach temperature confirms the fan restoration corrected the problem.
In class: Field walk to school's rooftop or parking-lot condensing unit; students measure ambient air temperature, condenser inlet and outlet air temperatures, and discharge pressure, then calculate approach temperature and record on a data sheet. · Instructor demonstration of fan cycling control and fan speed control (VFD) on a training rig; students adjust thermostat setpoints and observe effect on head pressure, recording data at each setpoint. · Group problem-solving exercise using a case study of a condensing unit short-cycling in cold weather; students identify the cause and select the correct head-pressure control remedy from a provided list of options.
Materials: Rooftop or remote condensing unit, digital thermometers, manifold gauge sets, clamp ammeter, head-pressure controller training rig, data recording sheets, ASHRAE Refrigeration Handbook condenser chapter
Check: Data sheet reviewed by instructor; approach temperature calculation must be within ±2°F of instructor's measured value, and head-pressure control answer must be correctly justified. Pass requires both elements correct.

Practice check8

Answer each question, then check it. Nothing is recorded; this is your own practice.

Core Refrigeration Fundamentals / HVAC Excellence Employment Ready
1. What does approach temperature measure in a condenser evaluation?
NATE Core — Compression and Heat Transfer
2. An air-cooled condenser has elevated head pressure and a higher-than-normal approach temperature. The ambient temperature is normal. Which cause is MOST consistent with these findings?
NATE Commercial Refrigeration — System Diagnostics
3. Why does head pressure fall too low when a commercial refrigeration condensing unit operates in very cold ambient temperatures without head-pressure controls?
NATE Commercial Refrigeration — Controls and Electricity
4. A VFD (variable frequency drive) used for condenser fan speed control offers which advantage over simple fan cycling?
HVAC Excellence — Commercial Refrigeration Systems
5. An evaporative condenser's performance is most directly tied to which ambient air property?
NCCER HVAC — Safety
6. Before connecting manifold gauges to a rooftop condensing unit, which safety action is REQUIRED if you must open an electrical panel to check a fan motor?
NATE Commercial Refrigeration — System Measurements
7. A refrigeration technician measures R-404A discharge pressure at 140 psig on a cold day. The ambient temperature is 25 °F. Using an approximate saturation temperature of 70 °F for R-404A at 140 psig, what is the approach temperature?
NATE Commercial Refrigeration — Troubleshooting and System Controls
8. A condensing unit in a parking lot short-cycles repeatedly on a day when the outdoor temperature is 30 °F. No other faults are found. Which head-pressure control remedy is MOST appropriate?
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