Two types of fixed-bore metering devices are capillary tubes and _________________________.

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A metering device is the component in an air conditioning system that drastically decreases the temperature and pressure of the refrigerant moving from the condenser before it enters the evaporator. There are different types of metering devices, and each one has different ways of charging that are crucial to the HVAC systems’ operation. Metering devices are engineered to govern the flow of refrigerant to help regulate pressure. In a typical system, the core functions of a metering device is to feed refrigerant to the evaporator and provide a high side to low side pressure and temperature drop.

Two Types of Metering Devices

There are two main types of metering devices: fixed and modulating. We’ll look at both types to understand when they may be used. “The fixed orifice metering device, containing a piston, produces a steady flow of refrigerant into the evaporator coil. The orifice size in the piston determines how much liquid refrigerant enters the evaporator coil.” A fixed metering device has fixed operating conditions, meaning its operating conditions stay the same no matter what external factors are present. It processes a consistent amount of refrigerant as its parts are not adjustable.

To understand how a fixed metering device works, it can be compared to a leaking water pipe. When there is water inside the pipe, water moves within the pipe with pressure acting on it. However, when there is a leak, the speed of the water leaking outside the pipe is much slower than the water moving inside the pipe. This is because the pressure of the water inside the pipe is contained and the leaked water’s pressure is much lower than the pressure of the water inside. The same concept is used in a fixed orifice metering device. The high-pressure refrigerant liquid coming from the condenser is contained in the liquid line.

The fixed orifice metering device has a hole that the refrigerant can pass through. However, the size of this hole is smaller than the size of the liquid line. This setup is how the fixed orifice metering device lowers the pressure of the refrigerant.

 Types of fixed metering devices

There are two types of fixed metering devices: capillary tube and fixed orifice. “A fixed metering device can be a simple capillary tube, which is a two-foot-long spiral of copper tubing that is narrower than the liquid line feeding into it.” This tube is normally used as a system’s metering device because of its simplicity and low cost. A fixed orifice device is easy to change, so some technicians prefer working on systems that are simple. 

As weather affects the temperature and pressure of our systems, our systems change in the way they operate (harder and less efficient), but fixed metering devices cannot respond to weather. Due to their limited applications, fixed metering devices are found in small units and older systems.

Modulating metering devices

“Modulating” is simply defined as changing. A modulating metering device can change how it operates based on different ambient conditions. The ability to change makes modulating metering devices suitable for a wide range of applications. However, this feature may make them more expensive than a fixed metering device.

There are three types of modulating metering devices:

• Automatic Expansion Valves (AEV): This type has a hole to allow refrigerant to move through it and it controls the pressure. The hole size is controlled by a diaphragm and spring.
• Thermostatic Expansion Valves (TEV): Maintains the correct amount of refrigerant supply by matching the flow rate against the rate of refrigerant evaporation in the evaporator coil. 

• Electronic Expansion Valves (EEV): Based on sensor readings, it controls the amount of refrigerant electronically.

Modulating metering devices can respond to different weather conditions and are much more energy efficient, which is a great reason to use them more widely. Although units with modulating metering devices are more expensive up front, they are an investment and their advantages outweighing the cost in the long-run as they help reduce energy bills.

As a rule of thumb, the air leaving the evaporator will be about ____ than the air entering the evaporator.

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A service valve can be used to ____.

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A typical evaporator superheat reading at the evaporator outlet for a central air-conditioning system on a 95°F day would be ____°F.

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On a 95°F day, the R-22 refrigerant pressure on the high side should be about ____ psig.

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The liquid line temperature is used to determine the ____ of a condenser.

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To accurately add refrigerant to a system, a condenser ambient temperature of ____°F should be simulated.

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The instrument to use to check for a short in the compressor motor winding is a(n) ____.

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With a rise in outdoor temperature, the head pressure will rise.

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Superheat occurs in the condenser as well as the evaporator.

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The best way to tell if a compressor's discharge line is at the correct temperature is to grab hold of it.

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Subcooling occurs in the evaporator as well as the condenser.

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When checking the electrical circuits of an air-conditioning system, each parallel circuit should be isolated and checked separately.

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If a compressor has a partially shorted winding, it could run for a while, overheat, and shut off.

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Reducing the CFM through the evaporator will cause the head pressure to ____________________.

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On most gauge manifolds, the suction or low side gauge is located on the ____________________.

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Temperature difference is used to describe the difference in the inlet and _________ temperature of a heat exchanger.

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Two types of fixed-bore metering devices are capillary tubes and _________________________.

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Reducing the CFM through the evaporator will cause the amperage draw of the compressor to ____________________.

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Why do we measure temperatures at the evaporator rather than the suction pressure?

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Why should a technician visually inspect the equipment before installing gauges?

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