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Basic Residential A/C System-System Diagram
Basic Residential Heat Pump System - System Diagram
Basic Commercial Refrigeration System - System Diagram
Dial Stem Thermometers
While assisting fellow technicians through refrigerant side diagnostic procedures, it’s often identified that the technician is using a “dial stem thermometer” to evaluate refrigerant line temperatures. This is not necessarily the preferred test instrument to use when attempting to expeditiously evaluate accurate temperatures of refrigerant lines.
The primary reason is “limited point of contact” between the thermometer probe and refrigerant tube. An example of a typical dial thermometer used in the HVAC/R industry has a probe with measurements of 0.15 inches in diameter. There is usually a dimple located somewhere along the probe. The surface area between the dimple and the point is the thermometer’s active “sensing surface”. In this example the probe’s sensing surface length measures 1.75 inches. Given these dimensions, the probe has a “sensing surface area” of 0.82 square inches. Although this is almost one square inch of surface area, when attempting to measure the temperature a flat or curved surface, actual interface contact made is a very small fraction of this total area, if not only a “point” contact. The remaining surface area is sensing the temperature of what is in contact with it (example: air, insulation, tool, or body temperature). Given this fact, the displayed temperature is skewed, since the probe will not differentiate between the different temperatures sensed.
Another reason is speed. A technician is required to take many temperature samples at different locations. This cannot be accomplished expeditiously using the dial stem thermometer.
Most of today’s digital meters have connections for type-K thermocouples. Clamp-on thermocouples are available accessories and by far the most popular designs of today. Clamp-on styles offer the technician the capability to measure temperatures quickly and accurately. See your local branch for further details.
: Whatever you decide, never use an “Infrared Temperature Sensing” instrument for collecting refrigerant line temperatures. The target area of these guns depends on the distance from the target and will average everything within this circular target, fore and aft. Most technical support personal will not and should not use measurements taken with an infrared sensing instrument in their diagnostic disposition(s).
Brazing with Nitrogen
As the HVAC/R industry fully engages in the R410A / POE era, we should each be reminded of the importance of proper brazing techniques, specifically the use of Nitrogen during the brazing process. During any brazing operation, Nitrogen should be metered through the copper lines to displace oxygen from the interior while being subjected to the high brazing temperatures. This will eliminate oxide formation on the interior copper tube surface.
What’s the potential risk of not using Nitrogen? Oxides will form and adhere to the interior copper wall. Because POE oils are very good solvents, it will easily remove oxides, grime, and residual mineral oil. As these contaminates are freed and circulate through the system, they are eventually captured by driers, screens, and metering devices causing restrictions and induce premature wear to critical compressor components such as the bearings, scrolls, or valves. Why hasn’t this been a critical issue with R22 and mineral oil? The quick and simple answer is — it has been, however these systems are more forgiving because mineral oil is not a solvent.
As an industry, a high percentage of TXV failures are the result of restrictions induced by contaminated systems. Given these facts, we must ask ourselves whether we want the liability of unnecessary call-backs or do we want to install and service HVAC/R systems using “best practices”?
There are many excuses for not using Nitrogen, for example:
1) I’m only going to braze a couple of connections, so it won’t be a problem.
2) I’ll braze really quick, so oxides won’t form.
3) No one will know I don’t use Nitrogen, so why use it?
None of these excuses have any validity, so let’s choose “Best Practices”.
Know Your Charge Method
Technicians should verify the following prior to modifying system charge (refrigerant):
What refrigerant does the system use, i.e. R22 or R410A? (know before attaching gauges)
What metering device is used on the Evaporator Coil, i.e. TXV or Piston?
Validate proper Airflow (must be completed prior to validating system charge).
Air Flow Validation Methods:
1) Delta-T Method: CFM = --------------------
Where: QSensible = Volts * Amps * 3.413 (for Electric Heat) or Heat Output (Gas Furnace)
T = Supply Air Temperature (SAT) – Return Air Temperature (RAT)
2) ESP Method: Measure External Static Pressure (ESP) and reference to OEM Blower Table.
Note: The type of metering device dictates charging method, not the refrigerant used.
- If using a TXV metering device always charge system using the Sub-Cooling Method:
Subcooling (SC) = TSaturated – TActual
Where: TSaturated = Corresponding “Refrigerant Saturation Temperature” at a given Head
TActual = Actual “Liquid Line Temperature”
- If using a Piston metering device always charge system using the Superheat Method:
Superheat (SH) = TActual – TSaturated
Where: TSaturated = Corresponding “Refrigerant Saturation Temperature” at a given Suction
TActual = Actual “Suction Line Temperature”
Note: When validating proper operation of the refrigerant side of system, both the SC and SH should be obtained.
- TXV system: SC will define proper charge; while SH can validate proper modulating of TXV.
- Piston System: SH will define proper charge; however SC can be an indictor of potential issues.
P1 = P2 + P3
P1 = Sensing Bulb Pressure (Opening Force)
P2 = Evaporator Pressure (Closing Force)
P3 = Spring Pressure (Closing Force)
TXV failures are usually the result of:
1) Restriction (Screen or Metering Point)
2) Sensing Bulb Loss of Charge
3) Diaphragm Failure
In HVAC applications, a TXV will typically be preset to maintain 8°-12° superheat, referenced at the sensing bulb. For proper operation, liquid refrigerant must be available at the inlet of the TXV; hence the importance of adequate sub-cooling. If sub-cooling is inadequate, refrigerant will flash and capacity will be compromised.
Understanding the forces (P1, P2, P3) and how they influence TXV operation is crucial to properly diagnose and validate the valve. Before anything can be said of the valve’s integrity, superheat and sub-cooling must be known.