9.3 Methods for collecting input data

Provided data are documented during the handling of a problem, and accurate measurements are obtained, in most cases it is possible for an experienced engineer to evaluate the data without visiting the installation.

Service Pressure Gauges and Thermometer

It can often be hard to collect the desired data. The normal measuring equipment is a service pressure gauge and preferably some type of thermometer. Data are commonly collected using fixed gauges or by connecting a service pressure gauge to the system. After the expansion valve or other component has been adjusted, the consequences of the change are monitored. However, a much better method is to gather more information systematically. Again, it is important to collect as much information as possible on-site. Data should be collected over as short a time span as possible, and measurements should be carried out several times over a period of time to verify the stability of the system.

Service pressure gauges should be calibrated regularly and be of good quality. The data obtained from the service pressure gauges will have a modest accuracy, and will depend greatly on the "human factor". They should therefore be used with caution. The data can be compared with the calculated performance of a system to find discrepancies.

A thermometer with a sensor attached with heat transfer paste, aluminum tape and insulation should be used (see Figure 9.2) to increase accuracy. If possible, fixed thermometers should be compared with a known reference, e.g. the service thermometer should be mounted at the same place as the fixed one.

A good way to check thermometers is to check the temperature difference over a heat exchanger and then stop the system for a few minutes, allowing the pump to circulate. The thermometers on each side of the heat exchanger should stabilize at the same temperature.

General Data Loggers

Loggers make it possible to collect accurate data simultaneously from several points and then display them over a period of time. Many loggers are only able to collect temperatures. However, they still give important information because evaporation and condensing pressures can be measured indirectly.

The evaporation pressure/temperature is close to the temperature after the expansion device (the pressure drop in the distributor and evaporator should be compensated). The condensing pressure/temperature is close to the liquid temperature, when compensated for sub-cooling. Provided conditions and capacities do not change very much, fixed gauges or service pressure gauges can be used to "calibrate" these temperatures. Whenever possible, loggers that can measure both pressure and temperature should be used. This method will increase the accuracy of the data compared with data collected with regular service equipment.

Specialized Refrigeration Analyzers

Loggers are available with integrated software, specialized for refrigeration applications, that contains all the physical data for most refrigerants. These loggers collect all the relevant parameters, and intervals can be selected as desired. These analyzers are easy to use, because they are preprogrammed and can calculate and present all interesting data immediately on-site.

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