Air Cooled Condenser
The condenser is a length of tubing with fins placed perpendicular to the tubing. Figure 15-2 shows a typical air-cooled condenser coil. This type of condensing coil is used in all size units from an average window unit to a commercial system having a three digit tonnage rating. The amount of refrigerant circulated per minute has a direct bearing on the tonnage rating of the system. The tonnage rating of the unit will determine the physical size of the coil in an air-cooled condensing unit. In Fig. 15-2 the affected heat transfer from the coil to the ambient air is shown. You can easily see that the ambient air temperature will determine the heat rejection efficiency of the condenser coil. Air conditioning systems are designed for the areas in which they are to operate. For instance, in a certain geographical area the unit might be designed to maintain a 72-degree F. temperature in a conditioned space, at an ambient temperature of 95 degrees. As the variable ambient changes, so does the temperature of the conditioned space. Not going into the engineering of it, air conditioning systems have a design factor. The perfect operating temperature of a condenser for example would be 105 degrees F. If the ambient temperature begins to drop, the system begins to lose efficiency. In Fig. 15-3 a condenser cross section is shown. The air flowing over the coil must be controlled if the unit is to operate efficiently during low ambient periods.
Fan cycling is controlled with the use of a reverse acting high-pressure control. The switch closes on rise of head pressure. The cycling switch might not be enough in those areas where a cold prevailing wind blows through the condenser coil.
In the prevailing wind situation, face dampers are placed on the condenser coil and piped to operate with head pressure. The dampers shown in Fig. 15-4 are used on many commercial applications. The dampers move in proportion to the head pressure. The only time the damper is in its full open position is when the compressor is operating at its design temperature.
This is not important to residential installation, but for the light commercial usage it is. Just think about a store owner with a freezer full of meat, with a walk-in freezer that is inoperable due to a low ambient. Even in light commercial air conditioning this could be important in places like a banquet hall. In those areas where cold weather is a rare occasion, a quick and temporary fix might be a piece of cardboard placed in front of the condenser coil to block the wind; a small hole might then be cut in the cardboard to allow a small amount of air to pass through the coil so it will not get too hot. On the other side of the coin, if things are too hot, the same problem can occur. In this case, there could be a defective condenser fan motor that is not available for one reason or another. The proper placement of a water hose and a fine spray directed at the condenser coil can save the day for someone. It is some of these simple things that make the good technician better. I’ve always instructed students that most of the service call should be done mentally, before picking up a tool. Talking to the owner and thinking carefully during the diagnosis will save time and a lot of your energy. Thinking over the job first will enable the right tools to be selected and brought to the job site. I have watched many technicians almost walk themselves to death, back and forth from the service truck to get the tools they forgot. Pre-planning or pre-thinking can help eliminate the condition. Refrigeration and air conditioning equipment must operate at their design temperature and pressure, in order to perform their specific job. Table 15-1 shows an example of some operating pressure found in general air conditioning, (high-temperature refrigeration). The equipment listed in the table are air-cooled condensers with a DX (direct expansion) evaporator. I want to say once again that when replacing a part, when it has a specific type of function, the part replacement should be exactly like the part being replaced. This is a very critical point when replacing a fan motor or fan blade. For instance, if a motor that drives the condenser fan blade is being replaced, and the speed is less than the original, the unit might function alright on cooler days. When the ambient temperature rises, the fan will not turn fast enough to provide proper heat transfer. This will cause the unit to be less efficient on the hotter days of the year. Not only will it drive the owner crazy, it will drive the next service technician crazy trying to find the problem on the cooler day. There are certain relays and electrical parts that might be used in a replacement situation and still perform the same task as the original. Remember that in certain items within the unit, replacement parts must be the same. Another example would be replacement of a section of capillary tubing. If the exact I.D. (inside diameter) is not available, a different size and the exact length of the original won’t work. If the size is changed, so is the operating pressures which in turn affect the design factors of the unit.