Friday, May 6, 2011
Charing - Read the Fine Print!
There is an old saying that “the devil is in the details.” That is certainly true of refrigerant charging. There are so many details to keep track of. Outdoor ambient temperature, indoor wet bulb temperature, evaporator airflow, condenser pressure, evaporator pressure, suction line temperature, liquid line temperature, superheat, subcooling and approach. I am sure I left a few off, but that list is long enough to make my point: there is simply a lot to keep track of. The confusion caused by trying to make sense of so many variables can cause some technicians to look at only one thing, often they look at only the suction pressure. This is like covering your ears so you won’t hear the thunder. It does not keep you from being struck by lightning. It is true that no manufacturer specifies all those things in their charging information. However, many of these details are assumed so that system performance can be judged by one or two key indicators. The problem is that even a manufacturer’s charging chart will not be accurate if the conditions you have do not meet the assumptions used with the chart. You must read the fine print. Most manufacturers disclose their assumptions somewhere on the chart or in their service manual. A commonly specified condition is that the evaporator airflow is 400 CFM per ton and the return air temperature is 80°F 50% relative humidity. In our shop we have a packaged unit with a very simple charging chart. It gives the suction and discharge pressure for five specific outdoor temperatures. However, it is assuming standard airflow across the evaporator and indoor conditions that meet AHRI rating conditions – 80°F dry bulb 50% relative humidity. It is seldom that warm in our shop, so the pressures are usually lower than those specified on the chart. In the field, it is common for the indoor temperature to be above the AHRI rating condition, that is why you are there in the first place. You can expect higher pressures as a result. Superheat charts that list only an outdoor temperature have a similar problem. They assume an indoor temperature and humidity level. Even charts that specify an indoor wet bulb and an outdoor temperature are still assuming standard airflow across the evaporator coil. For that matter, all charging charts are assuming both coils are clean, that you have good airflow across both coils, and that all the refrigeration components are performing correctly. If any of these are not right, no charging chart will work. You should make a habit of checking airflow, air filter, and condenser cleanliness on every call. Then read the fine print to check any specified conditions used with that chart.