ACRx® Efficiency and Capacity Estimating Technology

What is Efficiency Index (EI) and Capacity Index (CI)?

• Efficiency Index (EI) is an estimate of an air conditioner's efficiency as a percentage of what its efficiency could be (while running under the same driving conditions) if it was serviced to the Benchmark Performance Standard.
• Capacity Index (CI) is an estimate of an air conditioner's total capacity as a percentage of what its capacity could be (while running under the same driving conditions) if it was serviced to the Benchmark Performance Standard.
• If a unit is running at the Benchmark Performance Standard, its EI and CI is 100%. If its performance is degraded because of a fault (e.g dirty condenser coil) such that EI is 80%, then it is running 20% less efficient than is could be if it were serviced and the fault(s) were fixed.
• Efficiency Index (EI) and Capacity Index (CI) are fundamentally innovative tools and exclusive patented technology. There has never existed, and currently does not exist any where else, a tool for making practical field estimates of an air conditioner's efficiency and capacity. These tools, for the first time, relate the impact of service decisions to the equipment owner's utility bill.

How is EI and CI displayed on the Service Assistant?

EI and CI are displayed in large font on the top left. Estimated current power, annual runtime, and annual energy cost savings potential are presented on the top right. Nominal operating parameters, displayed in the middle, characterize the unit with no faults. Parameters include design capacity and COP, annual runtime hours, and $/kWh. Set these parameters by pressing the "Edit" button. The diagnostic message is displayed on the bottom.

How does the Efficiency Index (EI) relate to my electric utility bill?

• EI can be used to directly estimate the impact of poorly performing air conditioners on utility bills. This can be done using more and less sophisticated techniques.
• The Service Assistant has a relatively simple, but useful and straight forward technique, integrated into it. This involves simply multiplying the utility rate (e.g. 0.10 $/kWh) times the average number of runtime hours per year (e.g. 1200 hours) times the energy consumption per ton (e.g. 1kW/ton for an average efficiency unit) times the nominal capacity of the unit (e.g. 20 tons) times EI estimated by the service assistant (e.g. 20%).
• In this example, a 20 ton unit consumes 24,000 kWh/year at a cost of $2,400/year. A 20% efficiency improvement on this unit is worth $480/year in energy savings.
• This is a building tune-up report for a big box retail store with energy savings calculated based on data collected before and after servicing each unit. The Service Assistant was used to survey many stores. ServiceAssistantOnline reports were used to identify this store, which was a strong candidate for improvement. Targeted service on many units provides about $20,000 in energy saving in one year for about $4,000 in service work (mostly coil cleanings and charge adjustments).

• Document efficiency and capacity during every standard inspection.
• Show the potential to save money in energy and compare to service cost to estimate both costs and benefits for doing service.
• Target units in a large portfolio that can most benefit from energy saving service procedures.
• Document the improvement in efficiency and capacity as a direct result of the values added by effective service (e.g. condenser coil cleaning).

• EER is the Energy Efficiency Ratio and is defined as the capacity measured in BTU/hr divided by the power input in kW. Federal government efficiency standards for larger units are defined in terms of EER under a standard "design" or "loaded" driving conditions. The standard since about 1990 has been 10 EER. High efficiency units are in the 12-16 EER range.
• SEER is the Seasonal Energy Efficiency Ratio and is defined as EER averaged over the driving conditions experienced by a unit over a standard cooling season. SEER, unlike EER, is able to capture the efficiency benefit provided by unloading a compressor (i.e. reducing its capacity) under low-load or non-peak conditions experienced by most air conditioners during the spring and fall or at night.
• COP is the Coefficient of Performance and is defined and the capacity divided by power input and both being measured in the same units (e.g. kW). It the same as EER except with different units. COP is used more commonly by engineers.

• Efficiency Index (EI) and Capacity Index (CI) were designed to provide a practical field estimate of an air conditioner's efficiency and capacity with reference to making better service decisions.
• Air conditioning service is an extremely cost sensitive business. Traditional laboratory measurement techniques involve expense equipment and substantial time to the extent that it is too impractical to implement on a large scale in the field. This is more true for measuring capacity than electrical power input. EI and CI balances providing useful information for better understanding the impact of proposed or completed service on a unit's efficiency and capacity with the cost of making the measurement. In fact, it errors on the conservative side of not burdening the technician with any additional measurements that are not already standard for basic diagnostics.
• Fundamentally, and for the first time, EI and CI uses compressor manufacturer's performance data in a large scale service application. This performance data, intended to be used by design engineers when applying a particular compressor in an air conditioning or refrigeration system, provides the refrigerant mass flow rate and compressor power consumption primarily as a function of suction and discharge pressure, which are both common field service measurements.

What aspects of equipment performance impact EI and CI the most?
EI and CI are mostly a function of evaporating and condensing temperatures. Fundamentally colder evaporators and hotter condensers cause air conditioners to run less efficiently and provide less capacity. Low evaporating temperatures are commonly caused by faults like dirty evaporator coils or restricted air flow and low charge. High condensing temperatures are commonly caused by dirty condenser coils and over charging. EI and CI help relate the readily apparent low evaporating temperature and high condensing temperature to their impact of operating efficiency and then directly to utility bills.

Does EI and CI help evaluate if replacing a unit with a higher efficiency is cost effective?
No. The performance standard used for comparison in EI and CI is the performance of the unit being worked on running under ideal or benchmark service conditions. It is not sensitive to the possible benefits of replacing the unit with a more efficiently designed model. However, this is not a fundamental limitation and therefore could by an extension to this technology that may be available in the future.

How does EI and CI work?

1. Quickly collect the standard Service Assistant data with the Quick Test temperature sensor array. This includes high and low side pressures, suction and liquid line temperatures, and condenser inlet air/water temperature.
2. Calculate the refrigerant flow rate and compressor power consumption under these measured conditions using a numerical compressor model.
3. Calculate the superheated suction line enthalpy (i.e. energy stored per unit mass of refrigerant) from the measured pressure and temperature.
4. Calculate the subcooled liquid line enthalpy (i.e. energy stored per unit mass of refrigerant) from the measured pressure and temperature.
5. Calculate capacity as refrigerant flow rate times the difference in enthalpy between the suction and liquid lines.
6. Estimate total input power by assuming fan power is a reasonable percentage (e.g. 20%) of the compressor power.
7. Calculate efficiency by dividing capacity by total input power.
8. Calculate the reference values of efficiency and capacity based on the Benchmark Performance Standard conditions.
9. Calculate EI and CI by dividing the efficiency (capacity) estimated based on the current conditions by the reference value of efficiency (capacity) and multiplying by 100.

What is the accuracy of EI and CI?
The accuracy of EI and CI are about 7 to 8 percent from the readings.

When to be cautious when using EI and CI?

• EI and CI requires estimates of the enthalpy (energy stored per unit mass) of the refrigerant in the liquid and suction lines. This is a straight forward calculation based on well known refrigerant properties for subcooled liquids in the liquid line and superheated vapor in the suction line. Under extreme fault conditions (e.g. severely under charged units) this may not be the case and EI and CI can not be estimated using the standard technique. Since repairing these large faults provide an opportunity to document extremely high efficiency improvements (>50%), we have developed a less accurate, but still very useful, technique for estimating EI and CI when there is no subcooling. Larger inaccuracies can be tolerated when the change in performance is much more substantial.
• Performance standards used to define indices like EI and CI and commonly established to make the reference value the highest possible efficiency or capacity. This makes it so these types of indices can not exceed 100%. The Benchmark Performance Standard does not have this property and therefore values of EI and CI can exceed 100%. For example, this can happen when the evaporating temperature (ET) is higher than the target value. This happens because benchmark value is established with more performance issues than just efficiency and total capacity in mind. The cooler evaporator has been generally specified for air conditioning applications for decades to provide adequate latent cooling capacity (too remove moisture for the air). EI and CI are not directly sensitive to latent capacity, but yet it was considered in the determination of the performance standard.
• Research has shown that using the exact compressor performance data (for the make and model being tested) is not essential because the same performance standard is used on the top and bottom of the performance ratio. In other words, EI and CI can be interpreted as "What is the efficiency or capacity of a reference air conditioner (using the selected compressor) running under the current conditions relative to the efficiency or capacity of that same reference air conditioner operating with ideal or benchmark performance under the same driving conditions". The difference between selecting different compressor, if is was available, is only a few percent.
• EI and CI estimates are based on the assumption that the compressor's performance has not degraded substantially since it was manufactured. EI and CI are only useful for estimating the impact of the other faults (e.g. too much or too little charge and dirty heat exchangers) on efficiency and capacity. One way to test this assumption is to measure the compressor's current and compared it to the values provided by the compressor performance data under the current conditions. If the current matches, there is reason to have more confidence that the compressor is performing as designed (useful in its own right) and that the flow rate and power estimates provided by the performance data are valid. This capability is not integrated into the Service Assistant yet.

• An air conditioner's "driving conditions" describes the environment a unit is placed in that completely defines how it responds such that if it were running under the same driving conditions at another time it will respond (e.g. all refrigerant pressures and temperatures) in exactly the same way.
• The primary driving conditioning for an air cooled unit is the outdoor ambient dry bulb temperature. The secondary driving condition for a DX (direct expansion) unit is the evaporator entering air wet bulb temperature. Wet bulb temperature better characterizes evaporator conditions when a unit is removing water (i.e. condensate) from the air. This is generally the case. Under low humidity conditions, when moisture is not being removed, evaporator entering air dry bulb temperature is more appropriate.
• Because they characterize the "operating environment" the unit is running in, outdoor ambient dry bulb and evaporator entering wet bulb temperatures and used by the "Carrier slide rule" to determine what the desired superheat should be under these conditions.

Symbol	Description
ET	Evaporating Temperature
SH	Suction line superheat
CTOA or COA	Condensing Temperature over Ambient
SC	Liquid line subcooling

• ET is evaporating temperature evaluated using the PT Chart and the suction pressure (SP).
• CT is condensing temperature evaluated using the PT Chart and the liquid pressure (LP).
• ST is the suction line temperature at the compressor.
• AMB is the outdoor ambient air temperature entering the condenser coil.
• LT is the liquid line temperature.

• An air conditioner's efficiency is defined as the ratio of what is produces (heat absorption rate into the evaporator) over its operating cost (electric input).

• An air conditioner's capacity is the rate of heat absorption into the evaporator. It is fundamentally the useful product produced by the machine.
  • Total capacity the the heat absorbed rate by the refrigerant and is the sum of the sensible and latent capacities on the air side. The capacity index (CI) refers to the total capacity of a unit.
  • Sensible capacity is the capacity used to reduce the dry bulb temperature of the air.
  • Latent capacity is the capacity used to remove or condense moisture out of the air without reducing the air's temperature.
  Capacity is commonly measured in "tons" of cooling. 1 ton of cooling capacity is equal to 12,000 BTU/hr (another common unit for capacity). Residential central air conditioners are usually in the 1.5 to 3 ton range. Commercial rooftop units are usually in the 5 to 20 ton range. Large chiller plant used in large buildings or campus and commonly in the 1000 of ton range.

What testing has been done to verify that EI and CI provide accurate and useful estimates of efficiency and capacity?
Laboratory testing of package rooftop units has been performed at Purdue University. Detailed laboratory quality efficiency and capacity measurements were performed in addition to the inputs required for EI and CI estimates under and larger selection of operating and equipment fault conditions. Comparisons have verified the accurate application of EI and CI under these large collection of conditions.