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PRESSURE TESTINGPerform a compensated pressure test with dry gas. After brazing, sweep the system with dry nitrogen to move out any noncondensables and push out any liquid (water) that entered the system during installation. (Believe it or not, just cooling a leaky fitting with a wet rag can cause this by capillary action if there is no positive pressure on the system).
Pressure test the system with nitrogen and a trace of refrigerant at the manufacturer’s prescribed test pressure. Using a digital gauge with temperature compensated pressure testing like the Testo 550 will minimize the time required and significantly increase accuracy by taking into account both pressure and temperature. A low-cost digital gauge like the Testo 550 utilizes high-accuracy temperature-compensated pressure transducers and separate air temperature sensors to perform a timed pressure drop test with ambient temperature compensation. The gauge simply makes all the measurements and does the calculations for you.
After the stabilization period, the technician starts the pressure test and the initial pressure, current pressure, and pressure difference are displayed. The pressure difference is where the rubber meets the road because it is the calculated difference based upon ambient temperature and Charles’ law. The initial and final pressure may be different with no change in differential pressure as the change in temperature is automatically compensated for. This, in conjunction with the higher accuracy, allows the technician to significantly reduce the time required to verify a leak-free system.
In practice, I have a very high level of confidence at half the required time. The industry standard for systems up to 10 tons is currently 1 hour; over 10 tons, the recommended time is a 24-hour period.
Whether or not you reduce your pressure testing time may be a question for the high pressure piping inspector, but a significant factor in pressure testing is the resolution of the gauges used to do it. Because the test is a timed test, an acceptable leak rate (if any) can be easily determined during testing. When venting the nitrogen again, do not drain the system completely. Leave 1-2 psi in the system isolated with the core tools and attach the vacuum lines to the system for evacuation.
EVACUATION1. Use large hoses. The deepest vacuum we can achieve is 29.92-inch hg or about -14.7 psig. We are limited by the physics of pressure. No matter how big the pump or how long you let it operate, it simply cannot achieve any deeper vacuum. So how do we increase flow volume if we cannot increase pressure? The answer is larger hoses.
Even when restricted by ¼-inch system ports, using a 3/8-inch or ½-inch hose greatly reduces resistance and increases potential flow. This is especially true during system evacuation. Just using one ½-inch hose with ¼-inch fittings can result in one-tenth the evacuation time compared to one ¼-inch hose. Think of it this way: You can reduce 60 minutes to 6 minutes.
2. Use vacuum-rated core tools and remove the valve cores. Valve core tools have several functions. First, removing the valve cores allows for full, unrestricted flow at the service valves. Second, the valve core tool allows for the isolation of the hoses from the system when testing the ultimate level of vacuum. All hoses leak. Removing the hoses and the manifold from the system during testing minimizes the potential for chasing non-system related leaks.
3. Use clean dry vacuum pump oil. Clean oil saves time. Moisture, dirt, and other system contaminates decrease pump performance. Some new synthetic based oils, like the Appion Micron-Dry, are designed to be hydrophobic by nature. This means that while the oil does help to draw the moisture from a system, it does not tightly bond with the moisture. Starting every job with fresh, clean synthetic vacuum pump oil can save you time on the evacuation - and money on your pump maintenance.
AIRFLOWSet airflow with a vane anemometer. There are many methods to set airflow; one of the fastest and most accurate for any residential system is a vane anemometer.
The mini-vane anemometer is an ideal tool to measure airflow in a duct, across a heat exchanger or evaporator coil, as required in the commissioning process. The mini vane allows for a full duct traverse with an automatic calculation of the cfm in the duct (as the duct dimensions and the free area are input into the instrument before the measurement is taken). If done carefully, the measurement error will be less than 3 percent, and often less than 1 percent error. Changes in yaw and pitch of the probe head in the duct by as much as 10 percent will result in less than 1 percent error in the measurement, making the mini-vane an ideal probe for field in-duct air measurement. Because of the larger probe size compared to a Pitot tube tip or hot wire bead, stray eddy currents will have little effect on the final measurement.
At the grilles/registers, a 4-inch-diameter vane anemometer may be used to determine air velocity leaving at each terminal outlet. Again, no air density compensation is required; it is a simple one-handed operation. Another advantage is a more accurate average of true airflow over the sample area. Many times four to six service trucks can be equipped with a large vane anemometer for the equivalent cost of one of the more expensive options previously mentioned.
REFRIGERANT CHARGING1. Use digital gauges. Digital instruments are faster, more accurate, more reliable, and have a higher repeatability than analog tools. Digital instruments stay in calibration, allow trending, allow more complex functions, and save time. Digital instruments allow data to be recorded and reported without human error, and provide reliable and accurate results for you and your customers. Data can be recorded much faster than any technician could ever do the calculations, and data can also be recorded whether or not the technician is there to see it. In most cases, the data is an uneditable record, so what you see is what was measured at the jobsite.
2. Charge directly by superheat or subcooling. Digital leaves no room for interpretation; it is what it is. With digital, you will find yourself setting up the equipment exactly to the manufacturer’s specifications because you can. If the manufacturer calls for 9°F of subcooling, you can charge the system to exactly 9°F. There is no learning curve beyond learning to navigate the menus of the analyzer. Simultaneous measurement of superheat and subcooling, with dual temperature sensor porting standard, allow instruments like the Testo 550 to be used with a second optional probe for charging and troubleshooting without the need to move the temperature sensor. Watching both sides of the system while charging allows the technician to spot problems like a faulty expansion valve at system start-up before the system is overcharged. When performing any diagnostic testing, both sides of the system must be evaluated before adjustments to the charge or mechanical repairs are made.
MEASURING WET BULB TEMPERATURESKeep your socks in the drawer. Using a cotton sock and thermocouple is a less-than-advantageous way to measure wet bulb temperatures. While it may appear to be a cost-effective solution, it can be plagued with errors, including incorrect air velocity across the sensor, changes in water evaporation rate from sock contamination or non-distilled water, or simply the sock drying out during testing. While it can produce accurate readings when done properly, it’s worth every penny to use a digital solution. As an example, a digital psychrometer like the Testo 605 H2 is fast, accurate, repeatable, and robust.
PERFORMANCE TESTINGAs an industry, we sell capacity yet we very rarely measure it. Now that we have a way to make accurate airflow measurements and quickly determine the change in wet bulb with a digital psychrometer, it is very easy to determine the capacity (Btuhs, kW or tons of cooling) a system is producing. Using an instrument like the Testo 435 equipped with a vane probe and two wireless sensors makes quick work of what might otherwise seem a tedious process. While field calculations can provide a reasonable estimate of the capacity of a system, the Testo 435 can quickly and accurately measure the true airside performance of the system, including all the corrections required for non-standard air.
USE THE RIGHT TOOLSWhile it may be impossible to do it more quickly the right way than the wrong way, you can cut considerable amounts of time and add considerable amounts of profit to several processes your technicians should be performing in the field.
So avoid the callback and the warranty claims. Avoid the repercussions of getting fined by the EPA. Even if the chances are small, fines of $12,500 per day per violation are incentive enough for me.
The right way does not have to be time prohibitive. You can do it right in a lot less time with the right tools and processes, and see additional savings on the back end of every call.
Publication date: 01/31/2011