Compressor Efficiency

Efficiency in a drive machine is the ratio of input energy between output work/(useful energy), between input energy (heat).

When we talk about how well things are done, we mean indirectly how efficiently or inefficiently we do them. A drive machine will be more efficient if it does its job in less time and with the least amount of energy supplied. Thermal drive machines require an amount of energy to be able to perform a job such as fuel that is transformed into heat energy in the case of a car internal combustion engine. In the case of a cooling machine the cycle is reversed, it is required to give it an amount of work to move an amount of thermal or heat energy. Every thermal machine, whether motor or cooling, requires giving up undoubtedly an amount of heat to the environment, so its operation is governed by our mother nature or the environmental conditions that surround it. The same refrigerator or air conditioning system operates very differently in Alaska or in the Middle Eastern Desert, in Mexico City or in the city of Yucatan, even the same equipment does not work the same in different environmental conditions or times of the year (in winter and in summer).

Efficiency in a driving machine is the ratio of input energy between output work (useful energy), between input energy (heat). In a refrigeration or cooling machine, it is the ratio of the inlet heat or energy (cooling), between the work or input energy required to produce the cooling. Fig. 1.

Energy given to the Cooling System in this case Electric Power to the engine (Watts)

Q_H = Output Energy or Heat rejected to the Environment (Btu/h)

W = Q_H – Q_C Compressor Input Work (Watts)

Q_C = Heat of entry into the Evaporator (Btu/h)

Efficiency = Q_C / Q_H – Q_C = Q_C / W (Btu/h – W)

In this article we will dedicate ourselves in principle to the study of the efficiency of cooling machines or systems (Air Conditioning and Refrigeration).

Concept of the Energy Efficiency Ratio (in English, Energy Efficiecy Rating EER). In relation to Fig. 1 , the most efficient cooling cycle is the one that takes the most QC heat from the machine, with the minimum consumption of mechanical work of the compressor (minimum electrical energy). Therefore, the efficiency of a refrigeration machine or Energy Efficiency Ratio is defined as the ratio of QC between W, in units Btu/h-W, or Kcal./h-W.

Since the Watt-h is a unit of electrical energy, equivalent to the heat energy of 3,413 Btu, the efficiency of a refrigeration machine can also be expressed in terms of W/W (value without units, expressed per unit p/u) which is called the Coefficient of Performance COP.

Coefficient of Performance COP; its value is p/u, ratio of the Thermal Power in Watts, between the Electrical Power of input to the Compressor Motor in Watts.

In Refrigeration; COP, Cooling Thermal Power in Watts (Qc cooling) / Input Power (W).

In Heating or Thermal Pump; COP, Heating Thermal Power in Watts (QH Heating) / Input Power (W).

Within the Refrigeration Cycle as far as efficiencies are concerned, the compressor is perhaps the one with the most consideration, it is affected by the following losses or inefficiencies:

Compressor Losses

Mechanical Efficiency: It is the one that takes into account the losses of mechanical power, (losses due to internal friction, lubrication, lubrication pump, etc.). The use of specified oil, with its suitable viscosity, lubricity, solubility and miscibility characteristics with refrigerants, are decisive in reducing friction losses.

Volumetric Efficiency: Mainly due to the reexpansion of the refrigerant gas inside the cylinder, the leakage of the gas by valves and piston rings (blowby), obstruction or loss of load of the flow of refrigerant gases, these losses are a function of the compression ratio in which the compressor operates and the operating temperatures of the cycle.

Electrical Efficiency: It is due to the fact that the losses happen in the electric motors, the largest of these is in the copper, due to the losses by Joule effect P = I2R, (proportional to the square of the operating current of the motor by the electrical resistance of its windings). Better efficiency designs use larger diameter conductors, to accommodate them inside the motor it is required that it be larger with larger grooves, also increasing the number of laminations. Other losses are losses in iron and laminations, which are divided into Losses by Eddy or Eddy Currents, which are produced proportionally by the magnetizing inductive effect of the current and its electrical frequency. and Hysteresis Losses caused by magnetization and demagnetization of the motor rotor and stator, which are a function of electrical frequency and lamination materials. Decreasing the thickness of the laminations, electrically insulated between them, and suitable iron alloys considerably reduce these losses. Larger rotors help decrease these problems, but laminations are the main factor for their reduction. Most commonly used engines use carbon steel laminations, of thicknesses of the order of 0.025 pg with losses of approx. 6 Watts per kg. High efficiency motors use Silicon Steel laminations with thicknesses of the order of 0.018 pg with losses of the order of 3 Watts per kg. Additional is the isolation of the laminations. In addition to reducing magnetic losses in High Efficiency motors their operating temperature decreases, and the life of the motor is increased, for every 10 °F of temperature reduction the life of the engine and its windings is doubled.

Thermal Efficiency: Due to heat losses in the compression of the gas in the compressor. Because compression is not perfectly adiabatic. The high temperature of the compression causes heat to dissipate through the body and walls of the compressor cylinder, it is energy or heat lost to the environment.

Refrigerant Efficiency: Their pressure-temperature characteristics, their heat transmission coefficient, their adiabatic coefficient, etc.

The efficiency of the cooling cycle is variable, it depends on the parameters that govern it in its two temperatures (and corresponding pressures), as is well known a thermal machine must always operate between two temperatures. In our case of Evaporation Temperature and Condensation Temperature, both are continuously variable.

The evaporation temperature varies according to the application of the conditions of the product to be cooled, its inlet temperature, its rotation frequency, relative humidity, cooling speed, thermal load, etc.

The Condensation Temperature will depend on the ambient temperature conditions; summer, winter, etc., and the environmental conditions of the place; Mexicali, Federal District, Yucatan, Alaska. If the ambient temperature increases by 10 °F the condensation temperature will tend to increase by the same amount by 10 °F, and vice versa: the temperature (and its corresponding saturation pressure), are a function of the ambient temperature.

Fig. 2 shows some of the operating values of a Discus Semi-hermetic compressor.

For example, when taking some capacity value (red color) you have to at 130 °F of damning to 10 °F of evaporation, a capacity of 45900 Btu/h and an input power (blue color) of 8300 W. However, if the conditions vary to 120 °F of condensation and 10 °F of evaporation, you have a capacity of 51000 Btu/h and an input power of 7950 W.

It is observed that a reduction in condensate temperature (or ambient temperature) of 10 °F leads to a capacity increase of 11.1% and a reduction in power consumption of 4%. Figures 3 and 4 show us the operation of two systems, the first Fig. 3 in which the condensation temperature is kept artificially high in order to keep its pressure high to maintain the required refrigerant flow in the expansion valve (with fan cycling, reduction of air flow, discharge pressure regulating valves, etc.). (Note that this high design temperature lasts a few days a year, as an example see Fig. 6.)

With the use of balanced port thermostatic valves from Emerson Climate Technologies, the pressure drop effect on the condenser does not mostly affect the flow of refrigerant over time, as it follows environmental variation or nature (Fig. 4), so you can work with lower pressures, leading to a higher capacity, better efficiency and operating savings.

For several years the published capacity values and EER efficiency values have been based on the ARI standards according to the following:

The above conditions were used for many years in order to meet the most unfavorable conditions, without taking much into account the energy savings, which today it is imperative to contemplate, so now in addition to meeting extreme operating conditions, it is necessary to efficiently optimize the compressors in real operating conditions.

From Fig. 5 it is obvious that all Air Conditioning (and Refrigeration) systems operate very little time in the conditions of high ambient temperature.  Most cooling systems or their longer utilization time lead to condensate temperatures below 130 °F (54.4 C).

Fig. 6 shows us the variation of the ambient temperature in Mexico City, a similar and very different graph must exist for each place; Monterrey, Acapulco, Rio de Janeiro, Yucatan, Veracruz, etc.

As you can see the range of ambient temperatures in Mexico is below 54.4 C (condensate), so considering a system with this temperature would be very expensive in its operation.

With the above there is the question: what is the COP or the EER? For its determination it is necessary to consider it on an annualized basis, and determine the ACOP and the AEER as we will see below.

The real values taken from the tables of operation, capacity (in Btu / h or Watts) and its required power in Watts, of the compressor that you want to obtain the ACOP and the AEER, and with the values of the graph of variation of the ambient temperature of the place that you want to know the values are considered, in this example the values of Fig. 6 are taken. As you can see the ACOP value obtained that would be the representative of an entire year in Mexico City compared to the value at 130F (54C) of 0.99 which means practically nothing.

It is important to mention some terms used in relation to the efficiency of compressors and cooling systems mainly in air conditioning, and refer mainly to seasonal operation.

SEASONAL EER (Seasonal Energy Efficiency Rating)

SEER = Total Cooling in one Year between the Total Energy Used in the Year.

Heating Seasonal Performance Factor

HSPF = Total Heating in one Year between The Total Energy (Including Auxiliary Heating) Used in the Year.

SEASONAL PERFORMANCE FACTOR

APF = Total Heating and Cooling in the Year among the Total Energy Used in the Year

See original.