Air is known as the enemy of any refrigeration system. Purging, whether manual or automatic, removes air and maximizes refrigeration system performance.
Air in a refrigeration system robs it of its capacity to function, and failure to remove such air can be costly in terms of operating efficiency and equipment damage. Such damage is especially notable in the industrial-sized refrigeration systems commonly used in major cold storage facilities, food processing plants and some chemical plants.
Regardless of whether a system is charged with ammonia or a Freon refrigerant, the thermal efficiency of such systems will greatly improve when undesirable noncondensable gas (air) is removed. The process of removing air, which is colorless and odorless, is called purging. Over time, this process has become increasingly automatic. But, it is important to understand why, where and how to purge the system before attempting to rely on an automatic purging system. Figure 3.34 shows an industrial air purger unit.
Air can enter a refrigeration system by several places:
• When suction pressure is below atmospheric conditions, air can enter through seals and valve packing.
• Air can rush in when the system is open for repair, coil cleaning or adding equipment.
• Air can enter when the refrigerant truck is charging the system or when oil is being added.
Therefore, the accumulated air has negative impact on the system performance which can be summarized as follows:
• This accumulated air insulates the transfer surface and effectively reduces the size of the condenser. To offset this size reduction, the system must work harder by increasing the pressure and temperature of the refrigerant. Therefore, removal of air as quickly and as efficiently as possible is essential.
• Air in the system can result in excess wear and tear on bearings and drive motors and contribute to a shorter service life for seals and belts. Also, the added head pressure increases the likelihood of premature gasket failures. It can also decrease the power cost to operate the compressor by about 2% for each 1% reduction in compressor capacity. Thus, it is essential to choose the proper size and type of purger for the job.
The easiest way to determine the amount of air in a refrigeration system is to check the condenser pressure and the temperature of the refrigerant leaving the condenser. Then, these findings should be compared with the standard temperature-pressure for that particular refrigerant.
Example 3.4
If, for example, the ammonia temperature is 30°C, the theoretical condenser pressure should be 1065.2 kPa. If your gauge reads 1199.7 kPa, the excess pressure is 134.5 kPa. Under this condition, the power costs increase by 10% and the compressor capacity decreases by 5%, as determined by the per kWh cost of energy. As an example, if the pressure is reduced by 20 psi and the cost of electricity is $0.05 per kWh, the annual savings will be more than $2600 per 100 tons.
