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issue: September 2005 APPLIANCE Magazine

Engineering - Air Conditioners
Case Study to Achieve 13 SEER on a Split A/C System


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by Vijay Bahel, lead engineer, and Robert Lehman, managing director, Emerson Climate Technologies, Design Services Network

The following paper investigates the use of a simulation model to achieve a 13 SEER rating on a split air-conditioning system while reducing system design time and cost.

Presented are the findings of a case study in which Emerson Climate Technologies’ System Design Simulator, a steady-state heat pump model, was used to evaluate several design options rather than implementing the changes incrementally in a laboratory and evaluating the results of each change.

Simulation software has proven to drastically reduce development time and cost by limiting the need for expensive and time-consuming laboratory testing. Furthermore, this particular Windows™-based program proved to be user-friendly as well as highly accurate.

The goal of this study was to improve the performance of a nominal 10.5 SEER split air-conditioner to achieve the 13 SEER minimum required by the U.S. Department of Energy for units produced after January 2006. Targeted actual performance testing was used to validate the modeling tool during this re-design exercise.

Figure 1. Searching for Compressor Model to Match Baseline Compressor Capacity

Methodology

Several design options that may improve SEER performance were identified. The options evaluated include the following:
• Change the refrigerant from R-22 to R-410A
• Change the compressor from reciprocating to scroll technology
• Change the evaporator fan motor from permanent split capacitor to brushless permanent-magnet type
• Optimize the condenser exit subcooling and evaporator superheat
• Change the condenser fan motor to a permanent split capacitor model
• Change the flow control device from a fixed orifice to a non-bleed thermostatic expansion valve

It is important to note that for the purpose of simplifying this exercise, several other design changes were excluded from this analysis, including changing the geometry of both the condenser and the evaporator, as well as evaluating what impact any changes in tubing wall thickness might have on overall system performance.

The starting point was a nominal 10.5 SEER split air-conditioner using refrigerant R-22. Details of the base system are shown in Table 1 and its nominal performance is shown in Table 2.

The system simulator has a built-in performance database of more than 10,000 compressor models that may be used in finding an appropriate compressor to model. Further, the program’s “Update” feature helps ensure users have the most current performance data available by allowing them to update the program via the Internet. Figure 1 shows an example of the criteria used in finding the compressor candidates for this analysis.

This modeling effort required approximately 8 hr of engineering time to set up the system configuration and run the various cases. The model automatically calculates all parameters at each system state point and displays them in a convenient graphical format summarizing the key system parameters: capacity, efficiency, power, refrigerant charge, mass flow rate, pressure, and temperature and enthalpy at all state points of the cycle. The results of the baseline system simulation are shown in Figure 2.

Figure 2. System Performance Summary of Baseline R-22 Unit

Gains From Compressor and Refrigerant Changes

In order to identify the best replacement
R-410A compressor candidate, three scroll compressor models with different rated capacities and efficiencies were evaluated using the chassis of the base R-22 system. The search was limited to scroll compressors because they are inherently more efficient than reciprocating compressors. The compressor models used in the evaluation along with the baseline compressor selection are listed in Table 3.

During the first iteration, the refrigerant was changed from R-22 to R-410A since it has a significant heat transfer advantage over R-22 in both the evaporator (+35 percent) and the condenser (+5 percent) resulting in about 5 to 6 percent higher system Coefficient of Performance (COP) for an equivalently sized system. Other advantages, such as lower pressure drop, low temperature glide, and less refrigerant charge give refrigerant R-410A a clear advantage over other environmentally acceptable R-22 replacements.

After the system model identified the best compressor candidate, a validation test was performed in a psychrometric test room to confirm the findings of the system model before proceeding to evaluate any other design options. The validation test proved that the model provides an excellent correlation to the actual performance with the SEER rating correlating within 2 percent. A comparison of the modeled performance versus the tested performance is shown in Figure 3. Changing from a reciprocating to a scroll compressor and simultaneously from refrigerant R-22 to R-410A provided a 6.1-percent improvement in the system efficiency (EER).

Figure 3. Validation Run Showing Measured Vs. Simulated Results

Gains from Brushless Permanent Magnet Evaporator Fan Motor

Next, the existing permanent split capacitor (PSC) evaporator fan motor was replaced with a higher efficiency brushless permanent magnet (BPM) motor. BPM motors can offer about 15 percent higher efficiency relative to PSC motors and can maintain efficiency over a wide operating range resulting in improved system EER.

The system model’s parametric analysis capability was used to optimize the evaporator airflow rate. Keeping all other factors the same, airflow was varied from 65 to 100 percent, and the resulting system efficiency calculations automatically plotted (see Figure 4). The model predicted that the airflow should be lowered to 70 percent for optimum system efficiency. This design option utilized BPM’s capability of maintaining higher efficiency while using significantly lower motor power at the lower airflow rates.

The blower strategy takes advantage of the scroll compressor’s nearly flat compressor power input characteristic over a wide evaporator temperature range (see Table 4), along with a marked reduction in the fan motor power input when its air flow rate is lowered to 70 percent of the full air flow rate (see Figure 4). The 30-percent reduction in airflow lowers system capacity by 4.8 percent while the total system power goes down by 9.7 percent, thus providing a net efficiency gain of 4.9 percent. The efficiency of the BPM motor replacing the PSC motor results in an improvement of 2.9 percent in EER. Therefore, the overall system efficiency gain from switching to a BPM motor and reduced evaporator blower strategy was 7.8 percent. Table 5 shows the simulated results of these incremental changes.

The evaporator fan control strategy may be implemented by sensing the outdoor air temperature. In this control scheme, the blower control operates the evaporator fan at full airflow rate at the capacity rating point of 95°F outdoor air temperature. At the efficiency rating point of 82°F outdoor air temperature, the blower speed is reduced to 70 percent to take advantage of lower fan power.

An additional benefit of the blower strategy is improved dehumidification with a lower sensible heat ratio. The moisture removal capability is enhanced by 11 percent due to a lower evaporator dew point temperature (lower by 3.5°F) resulting from reduced airflow of 70 percent (see Table 4).

Figure 5. Scroll Compressor Power Input Characteristics and Total System Power

Gain from Higher Efficiency Condenser Fan Motor

The modeling tool identified a gain of 2.3 percent in system efficiency by switching to a higher efficiency PSC condenser fan motor. The expected performance is summarized in Table 5.

Gain from Optimizing Condenser Exit Subcooling and Evaporator Superheat

Several simulation runs were made using the parametric analysis capability of the system model to identify the optimum evaporator superheat and condenser exit subcooling combination. Lowering the condenser exit subcooling from 19°F to 12°F and evaporator superheat from 16°F to 10°F provided an efficiency gain of 1.6 percent.

Even though a lower superheat setting would provide better performance, 12°F superheat was selected to ensure stable operation of the thermostatic expansion valve (TXV). These optimization runs resulted in a system refrigerant charge reduction of ~6 percent. The gain from this design change is shown in Table 5. The model provides several built-in parameters for conducting the parametric studies to allow engineers to quickly optimize their system designs.

Gain from Non-Bleed Thermostatic Expansion Valve

A non-bleed TXV was chosen to replace the existing orifice. This improved the cyclic performance and provided an efficiency gain of 5.3 percent. The coefficient of degradation, Cd, for the base system equipped with orifice was 0.18, and Cd for the upgraded unit with TXV was 0.08.

Final System Configuration and SEER

Table 5 also shows the performance and the configuration of the final system. The 13.0 SEER was calculated using the simulation model’s built-in SEER calculation tool, which is programmed according to the U.S. Department of Energy’s procedure. Figure 6 shows the inputs for computing the SEER computation.

Engineering Cost Savings

Using this simulation software tool to model the system and analyze numerous design changes eliminated several weeks of laboratory testing and evaluation. While the real cost of engineering time varies by organization, it can safely be shown that there was a significant cost savings associated with using this tool.

Perhaps of equal importance is the potential time savings associated with using simulation tools to streamline the product development process and speed the time it takes to get new products to market. Getting the right product to market is important, certainly. Controlling and reducing costs are important as well. However, being first to market with a new and differentiated product can provide a significant competitive advantage and deliver lasting financial performance to the organization that can “win” this race on a consistent basis.

Conclusion

Efficiency gains of 23 percent were identified for boosting the baseline 10.5 SEER system to achieve the 13 SEER target. The breakdown of various efficiency gains is provided in Figure 7. All improvements identified may be implemented without a major hardware upgrade.

 

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