In our previous HVAC Journal entries, we reviewed the fundamental concepts surrounding heat, pressure, matter, and energy as they pertained to their functional roles in the HVAC/R industry. Now, we will be delving into how these conceptual laws are integrated into the day-to-day operation of what we do as technicians in the field. Let’s begin with refrigeration – what it is, how it’s used, and why it works the way it does.
Refrigeration is, at its most basic level, a process of removing heat from a specific place where it is not wanted, such as inside a home or walk-in cooler, and transferring it outside into the atmosphere. Prior to the advent of Freon®, refrigeration was a simple block of ice and the amount of refrigeration was based on how long it took to melt one ton of ice in a given period of time. Specifically, one ton of refrigeration is the amount of heat necessary to melt 1- ton of ice within a 24-hour time frame. What this equates to is 288,000 Btu to melt 1- ton of ice in 24-hours. Broken down even further, this means that it would take 12,000 Btu to melt this same amount of ice in 1- hour, or 200 Btu in 1-minute.
As it relates to preserving perishable food with refrigeration, it is important to note that bacterial growth that causes food to spoil slows down significantly at lower temperatures. As you can see the refrigeration cycle in action in the infographic to the left, refrigeration is sub-divided into three primary ranges depending on the associated temperature with the product in question.
The relationship of vapor pressure and boiling point temperature is aptly known as the pressure temperature relationship. One of the most important properties associated with this relationship is that the boiling point of water can be changed and controlled by controlling the vapor pressure above the water; however, in most closed systems, refrigerant is used in lieu of water since one pound of water at 40° F has a vapor volume of 2,445 ft³ compared to R-22 which is only 0.6 ft³. This would mean that the compressor would have to be very large to complete the same amount of work when compared to one required to compress refrigerant.
Regardless of which type of refrigerant is being utilized, they all share several characteristics; specifically, they must be safe, detectable, environmentally friendly, have a low boiling point, and ideal for pumping. For easier field identification, they are color coded to indicate the type of refrigerant in the cylinder. It is best to recover or store the refrigerant while the refrigeration unit is being serviced, then either recycled or reclaimed by the manufacturer as appropriate. A useful tool of the trade used to plot refrigeration cycles are enthalpy diagrams, which depict how much heat a substance has at some starting point.
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