How are compressor capacities determined?
This is the third part of a three part series I am writing on understanding compressor replacements. My goal is to relate the understanding of why compressor capacity changes at different system conditions and how to understand this when servicing compressors. The focus is based around questions I received at a recent tradeshow on replacing welded body reciprocating compressors with scroll technology.
There are several types of mechanical compression used in Air Conditioning applications available around the world today: rotary, screw, reciprocating and scroll just to name a few. As I stated earlier, my focus in this series will be on reciprocating and how this style of mechanical compression relates to scroll compression.
Back in the early 1970’s an industry organization named ARI selected a set of standards in which HVACR compressors would be rated with regard to their capacities. These “nominal” capacities would then be expressed per the ARI rating conditions; they are listed (ANSI/AHRI STANDARD 540-2004).
The purpose of this standard is to establish, for single and variable capacity positive displacement refrigerant compressors and compressor units: definitions; test requirements; rating requirements; minimum data requirements for Published Ratings; operating requirements; marking and nameplate data and conformance conditions.
When expressing capacities as they relate from one compressor to another, the rating points are referenced for a saturated evaporator and saturated condenser. For air conditioning applications this is typically spoken as “45°/130°”. To understand this clearly, it refers to a 45°F evaporating temperature along with a 130°F condensing temperature.
Looking at “Sheet A- R-410A Compressor Capacity,” provided with this article, we notice that this is a “Ratings Sheet” for a ZP25K5E –PFV compressor. Following across the top of the chart (X axis), it lists the evaporating temperatures along with the condensing temperatures down the left side (Y axis). This means that at a given condition, for both evaporating and condensing temperatures respectively, the chart expresses how the compressor is expected to perform at a given system condition.
Example: Sheet A- R-410A Compressor Capacity
If we follow a condition of 45°/130°, highlighted in yellow, we find that at this given system condition, this compressor is expected to produce 25,200 btu/hr. Understanding that the nomenclature of this compressor is a ZP25K, an R-410A scroll compressor. It should start to make sense that at these conditions, on this chart, you are looking at the ARI rated condition. If we move to a different condition, say 50°/100° (pink highlighted) – the compressor’s rated capacity has now increased to 34,300 btu/hr. By applying what we learned in part 1 & part 2 of this series, we know that things like compression ratios and volumetric efficiency play a huge part in how system conditions affect compressor capacities.
Example: Sheet B- R-410A Compressor Capacity
Opening “Sheet B- R-410A Compressor Capacity” we notice that this ratings sheet references a CP25K8E-PFV. This is also an R-410A compressor, but the type of compression is reciprocating (piston). If we follow the yellow highlighted conditions of 45°/130° we see that this compressor nominal rated capacity is 25,300 btu/hr. This number is also represented in the compressor nomenclature’s model number as “25K”. Following the 50°/100° (pink highlighted) conditions, the capacity changes with these conditions to 41,500 btu/hr. Compression ratios along with volumetric efficiency, greatly affect how system conditions relate to compressor capacities.
If you noticed, both of these R-410A compressor’s capacity ratings at 45°/130° are just over 25,000 btu/hr (2-tons). But because of their types of mechanical compression, along with the system condition compression ratios, we know that this affects the volumetric efficiency. As defined in part two of this series, volumetric efficiency (due to re-expansion gas) tends to have a greater impact on reciprocating compressors than on scrolls. Comparing the second conditions 50°/100° (pink highlights), we see that these capacities start to drift apart. For the scroll ZP25KE, 34,300 versus the reciprocating CP25K8E, 41,500.
Real World / Realistic Conditions
If a service technician was to replace the reciprocating compressor with a scroll, based solely on nominal (name plate) capacity, we see that at the same 50°/100° condition there is a difference of 7,200 btu/hr, or over ½ a ton of capacity. Does this make a difference? Could it make a difference? Since true compressor capacity is directly related to the actual system operating conditions, it is important to understand systems conditions, type of mechanical compression and rated capacities before making the service compressor replacement selection.
The good news, there are several Smartphone apps released by Emerson Climate Technologies which can help to solve replacement issues. Copeland “X-REF” app not only gives Electrical, Mechanical and Performance information, but it also identifies compressor replacements when looking to cross compressor mechanical compression platforms.