Low Ambient Controls

Key Points Overview

1. Definition of Low Ambient Conditions

2. Potential Risks in Low Ambient Conditions

3. Historical Control Methods

4. Modern Control Methods

5. Recommended Control Solutions

6. Additional Equipment Requirements

7. Installation Considerations

When in the course of human events it becomes necessary to run mechanical cooling when the outdoor temperature is low, low ambient controls are required to assure safe operation of the equipment. What do we mean by low ambient temperature? According to ICP installation instructions, below 55 degrees is low. Ambient refers to the air temperature surrounding the condensing unit. In other words, it is the temperature of the air passing over the condensing coil. Safe operation means conditions where it would not damage the equipment. Our primary concern is to prevent slugging of liquid refrigerant into the compressor.

In low ambient conditions, the sub-cooling portion of the condensing coil is sending ultra-cold refrigerant to the metering device. It is not completely turned into vapor in the evaporator. The hot gas returning to the compressor is saturated to the point of having liquid refrigerant droplets carried into the compressor.

One component that drives liquid refrigerant out of the compressor is a crankcase heater. Refrigerant likes to migrate to the coldest spot in the system. It is just as important to use a crankcase heater in hot conditions as it is in cold. Meant to move gas, not liquid, a compressor is not a pump. Hydraulic pressures from compressed liquid can reach as high as three thousand PSI on a piston face or in a scroll. A mechanical failure in the compressor is the result.

In the nineteen seventies, the only option was to turn the compressor off in low ambient conditions. Simple and inexpensive temperature switches did the job. An office with south and west exposure would still need cooling even if the outdoor temperature is too low to run the compressor. Turning off the condenser fan would allow the compressor to run while the ambient air did the cooling of the condensing coil. Turning the fan off by ambient temperature worked some of the time. A better approach is to cycle the fan based on head pressure or liquid line pressure.

The so-called peanut style head-pressure-fan-cycling control is inexpensive and easy to use. A swivel tee attached to the liquid line service port allows installation of the control while preserving the gauge port for service. These controls have a fixed range. For R-22 systems, the fan would turn off at about 125 psi and back on when the head pressure reached 225 psi. An R-410 system might use 300-400 psi. range. More sophisticated adjustable range controls are also available.

Fan cycling controls cause a lot of on/off. Wear and tear on a motor that starts and stops and the power consumed by starting a motor repeatedly are drawbacks to this approach. Controlling the fan speed improves performance.

Controlling the fan based on pressure works well but the condensing coil temperature also works. Johnson Controls makes a pressure sensing fan speed control. ICM and Hoffman are the leading manufacturers of temperature based fan speed controls. Ironically, the temperature-based controls are still referred to as head pressure controls. In fact, a thermistor embedded in the condensing coil is the sensing portion for these devices. Temperature-based head pressure fan speed controls are by far the most popular approach now. They are less invasive since they do not sense refrigerant pressure. They reduce the potential for a leak. They are also quite easy to set up because they have dials.

Some equipment manufacturers will also require additional component for low ambient operation. Evaporator freeze protection in the form of a bulb thermostat is one example. Some also require compressor hard start components. Putting these components on at the factory would be expensive. They would go to waste most of the time because manufacturers sell vast majorities of their condensing units in the south. We all have heard stories of people wearing winter coats when the temperature is below seventy. Low ambient controls in those climates are not a thing. Where we live running mechanical cooling below 55 degrees ambient is common. We should consider economizers whenever possible to avoid running the condenser at all in those conditions.

The application of these controls is relatively simple. The low ambient control interrupts the power supply to the fan either to stop the fan or to control its speed. Installing the type that measures coil temperature also requires placing the temperature sensor in or on the coil

By: Dallas Strong