Mastering the Art of Facilities Maintenance

managing time and resources effectively

Reading a set of gauges

One of the things with which new HVAC & Refrigeration techs constantly struggle is to learn to take the mystery out of reading a set of gauges and interpreting the information that they are seeing- but the trick is in paying attention to the fundamentals before trying to move on to more advanced diagnostics. Learn about the refrigeration cycle- memorize it, learn to draw it without looking at it- and don’t just draw the components, but be able to say what state (gas or liquid) the refrigerant is in at each point in the cycle.

Remember to practice safety principles- never let any refrigerant come in contact with your skin, inhale it, or get it in your eyes, flash freeze and chemical burns can occur. Also, never vent a refrigerant to the atmosphere, always recover refrigerant when opening or evacuating a system, and be aware that local, national, and international laws apply to the handling of refrigerants, and heavy fines can be assessed if you don’t know what you are doing. Most municipalities require that you take a refrigerant recovery test before handling refrigerant.

The whole point of any refrigeration system is the removal of heat. We are using scientific principles of heat transfer to remove the heat from one area (bedrooms, milk, chicken, buildings) and then reject it somewhere else (usually outside) Refrigeration does this in exactly the same way steam does it, it’s just that refrigerants boil at a temperature lower than water at atmospheric pressure.. It is generally referred to as the vapor/ compression cycle. In the evaporator, the refrigerant picks up heat and boils. this gas is then compressed, and cooled at the condenser, and sent back to the evaporator to boil again.

I know a few old rules of thumb, where techs have some calculation based on the outside air temperature, or something else- which may be enough to get the system running, but may be overcharged or still undercharged when outdoor conditions change.
The best thing to do, is to carry a thermocouple type thermometer, and a pressure-temperature chart and watch what the system is really doing. Remember that the point of any refrigeration cycle (no matter how complicated, or how new the refrigerant is) is all about heat exchange.Here’s a few things to consider:

When charging a system, take temperature readings of the system you are trying to cool. What sort of degree split are you trying to achieve? If you’re running and air handler, with a split system- the best you can achieve is a 15-20F or 8-11C degree split. If it is more or less than this- you are going to have problems down the road. The same thing is true with most walk-in boxes and rooftop units.

Have a look at your suction pressure, and the corresponding temperature. Once you begin adding refrigerant, and you start to get readings on the evaporator section, go back out and read your gauges. Does the suction pressure correspond to a temperature above 32 degrees? If not- is your system designed with a defrost timer or circuit board when appropriate. (A heat pump when charged properly will run a suction temperature lower than 32 degrees during some winter days)

Finally, learn to charge by measuring superheat and subcooling and using the superheat/subcooling method of charging. It is the best way to determine whether a system is properly charged. It requires that you take readings of the suction pressure and two temperatures — “the evaporator boiling temperature at a given pressure and the temperature of the refrigerant at the outlet of the evaporator on the suction line.” Print the linked article and carry it with you until you are 100% sure how to do it every time.

take some time to monitor how different systems run. Walk-in boxes that run on r-22, will have identical pressures/ temperatures to a residential R-22 system when they are first started (indoor temp of the box is 72F or 29C). Get in the habit of carrying a notebook and make cheat-sheets, taking notes when observing how different systems run. Pick up a copy of Modern Refrigeration (Amazon also has used copies) Remember-it’s all about temperatue change, boiling refrigerant, and the transfer of heat!


April 11, 2009 - Posted by | refrigeration, tools | , , , , , ,

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