Back to basics for leak-free, efficient systems

In this edited version of his paper, Back to Basics: Understanding Direct and Indirect Emissions, presented at the Institute of Refrigeration conference 2013, John Ellis of Ellis Training argues that the basic principles of RACHP systems must be understood if systems are to be safe, tight and efficient in their use of energy. 

Under BSEN378 all operatives’ involved in pneumatic testing must be sufficiently competent and knowledgeable to be safe.

Risk assessments and safe working procedures are often ‘generic’ having been drawn up in advance and too often operatives display a lack of understanding about the risks encountered when carrying out a high pressure pneumatic test. Pipe diameters and vessel surface areas are often not given any special consideration, also there is a poor understanding of what ‘pressure’ is and hardly any attempt is made to apply the relevant Gas Laws to the ‘pressure testing’ process.

It is common to see pressure testing records with ‘tightness test’ values considerably lower than the ‘normal running pressure’ never mind the HP switch or pressure relief valve settings. It is very rare to see how the testing operative took account of pipework temperature variation over the duration of the test.

Having conducted thousands of training sessions and assessments over many years it is very rare that people already know about the gas laws. On the contrary, many people erroneously believe that the reason that OFN is used is because its pressure does not vary with temperature. Consequently, they believe, a drop in pressure over the test duration must indicate that there is leakage and so many hours may be spent looking for a leak that may not be there.

According to the Gas Laws related to constant volume processes (ISOCHORIC):

So a tightness test being carried out overnight from an initial pressure of 34barg at 15ºC would be at 34.6barg if the temperature has risen to 20ºC and at 33.4barg if the temperature has fallen to 10ºC overnight.

Which begs the question how many tightness tests are carried out where:

a) there is a drop in pressure but no leak, ie because of a drop in temperature, the non-existent leak is ignored

b) there is a drop in pressure that does not relate to a drop in temperature but the leak remains undiscovered

c) the pressure is the same approximately at the end of the test but due to a temperature rise during the test the pressure should be higher and the leak goes unnoticed.

Evacuation

When the OFN has been released the system is evacuated and held below 2 TORR. A successful ‘vacuum rise test’ is often incorrectly considered to be the final indication that the system is tight. A tightness test should not be accepted unless it is carried out in accordance with BSEN378 and there is evidence that any pressure changes are in line with any temperature changes and it all should be recorded and witnessed.

Critically charged systems must be charged with the correct weight as specified by the manufacturer. For all other systems they must be charged so that all the major components can perform efficiently and then the system in total can perform efficiently over a wide variety of loadings and ambient conditions. It is worth emphasising that a tight system might not be an efficient system.

Indirect leak checking/performance testing

Indirect leak checking require a thorough knowledge of: the system type, application and design operating conditions; system refrigerant type and its properties and characteristics; vapour compression cycle as it relates to the refrigerant type and a thorough knowledge of alternative refrigerants.

A change of emphasis from operating pressures to operating temperatures is long overdue. In simple systems we should be talking about condensing temperature not discharge pressure and boiling temperature not suction pressure. We should also be talking about the condition of the refrigerant as it circulates the system to ensure that the system is always delivering the maximum cooling (or heating) capacity for the minimum energy usage.

Zeotropic blends

It is also essential that engineers have a thorough knowledge of zeotropic refrigerant blends and how some of their characteristics vary from single fluid refrigerants. Measuring superheat and subcooling are essential elements of indirect leak checking/performance monitoring and require an appropriate gauge set, surface temperature (pipe) probe and air probe and a refrigerant comparator or ‘app’ to establish saturated conditions.

Superheat with zeotropic blends is measured and calculated as the difference between the saturated vapour or dew point temperature and the measured temperature at bulb/sensor/compressor suction depending on the requirement.

Subcooling with zeotropic blends is measured and calculated as the difference between the saturated liquid or bubble point temperature and the liquid temperature measured at the inlet to the expansion valve.

Condensers need to produce liquid of good quality and at a pressure which is high enough to ensure adequate flow through the expansion device (even at low ambient conditions). Evaporator surfaces need to be ‘wetted’ with boiling liquid at the highest temperature that’s practical throughout their effective length.

For each one degree the ‘temperature lift’ between condensing temperature and evaporating temperature is increased the system compressor uses approximately 3% more energy and approximately 3% less heat is absorbed into the refrigerant. The lower the evaporator surface temperature, the lower the suction pressure, the less refrigerant is circulating and the lower the cooling capacity. If below 0ºC, the more frost forms, the harder it is to get the oil back to the compressor and the less efficient the system is.

In addition and finally, a plea to take more notice of the suction line during our routine service and maintenance. Unnecessary heat gain into suction vapour passing through the suction line leads to increased superheating which increases the specific volume of the refrigerant vapour and reduces the mass of refrigerant circulating. Less mass flow rate means less cooling capacity. Increased pressure drop has the same effect and so it is extremely important to check the condition of the suction line insulation and indeed that of the suction line itself. So suction line insulation should always be inspected as part of routine maintenance and its condition reported on as a priority.

This is an edited version of a paper presented at the Institute of Refrigeration Conference in London on December 2, 2013. Further information from the Institute of Refrigeration at www.ior.org.uk