Oxyvap Evaporative Cooling Applications Oxycom Fresh Air BV March 9th, 2015 Abstract This paper shows two highly efficient applications of the Oxyvap direct evaporative cooling technology, developed by Oxycom in The Netherlands: the IntrCooll and the PreCooll. 1 Introduction 1.1 Company overview Oxycom is a Dutch company specialized in the development and production of products and components for adiabatic cooling, indirect evaporative cooling, dew point cooling, air humidification and heat recovery. Oxycom developed, engineered and manufactures the Oxycell Indirect Evaporative Heat Exchanger and the Oxyvap Direct Evaporative Cooling pad. The Oxycell is the base of highly efficient all season fresh air systems that provide dew point cooling, indirect evaporative cooling, heat recovery and ventilation. The Oxyvap is the base of highly efficient cooling-only systems that provide direct evaporative cooling, humidification and ventilation. 1.2 Scope of the paper Following a previous publication [1] on the direct and indirect evaporative cooling principles, this paper focuses specifically on two highly efficient applications of the Oxyvap direct evaporative cooling technology. All values are expressed in SI units; American I-P units are shown in parentheses. 2 Direct Evaporative Cooling 2.1 General principle Direct evaporative cooling (DEC) is the most basic type of evaporative cooling. A supply airstream is brought in direct contact with a wet surface. The boundary layer around the wet surface is naturally saturated with water vapor. As unsaturated air travels along the boundary layer, water vapor will diffuse into the airstream, driven by a difference in vapor concentration. The water vapor content in the boundary layer is then restored to its natural saturated state by the adiabatic evaporation of water. The required latent heat for the phase change of water is taken from the sensible heat of the airstream, resulting in a lower air temperature. Figure 1 Standalone Oxyvap schematics 1. Hot and dry inlet air. 2. Cold and humid supply air. 3. Cold water returning from the Oxyvap pad. Oxycom Fresh Air BV - P.O. Box 212, NL-8100 AE Raalte - Phone: +31(0)572 349 400 - E-mail: info@oxy-com.com 1
2.2 Figure 3 IntrCooll schematics Efficiency The lowest possible temperature that can be achieved is the wet bulb temperature of the intake air, but is in practice somewhat higher due to the limited efficiency of the pad. The saturation efficiency or wet bulb efficiency is defined as the ratio of the actually achieved temperature drop to the maximum possible temperature drop. The cooling performance of the Oxyvap direct evaporative cooling technology has independently been verified by both TNO in The Netherlands [2] and CSIRO in Australia, showing 88.6% saturation efficiency at 2.0 m/s air velocity and 92.0% saturation efficiency at 1.6 m/s air velocity, both tested under inlet air conditions in accordance with Australian Standard AS 2913-2000 [3]. 1. Hot and dry inlet air. 2. Pre-cooled air. 3. Cold and humid supply air. Figure 2 Direct evaporative cooling process 4. arm and humid exhaust air. 5. Cold water returning from the Oxyvap pad. 6. arm water returning from the air-to-water heat exchanger. Chilled water from the Oxyvap is fed to the heat exchanger to pre-cool outdoor air before it enters the Oxyvap pad, that is then fed with heated water returning from the heat exchanger. From there, two parallel processes will take place. Air passing through the pad will be humidified and cooled towards its wet bulb temperature, but water will simultaneously release its heat as it strives to reach thermodynamic equilibrium with the wet surface of the pad. As a result, all air leaving the pad will have a relative humidity close to saturation, but at different temperatures. Since the system does not exchange energy with its surroundings, the cooling process as a whole will still be 3 IntrCooll adiabatic, just as in a conventional direct evaporative cooler, but the combination of the pre-cooling process 3.1 General principle and the recirculation of water between the components Conventional direct evaporative cooling technology, as will lead to a vertical temperature gradient in the air described in section 2, not only cools air towards its leaving the Oxyvap pad. wet bulb temperature, it also produces cold water, since The upper half of the supply air will have an average the stream of water is in thermodynamic equilibrium enthalpy higher than the intake air, whereas the lower with the wet bulb temperature of the airstream flowing half will have a lower average enthalpy. hen the upper through the pad. half of the air is discarded or diverted for other purposes, The IntrCooll is an Oxyvap-based cooling system, that the lower half can be used as supply air, reaching an averuses an additional air-to-water heat exchanger to en- age temperature below the initial wet bulb temperature, hance its performance. while less moisture is added compared to conventional direct adiabatic cooling. 2
Figure 4 IntrCooll cooling process 3.3 Impact The impact of the IntrCooll configuration compared to standalone Oxyvap technology has been calculated for the following conditions at the standard atmospheric pressure of 1013.25 hpa (29.92 inhg) [4] and at 2.1 m/s (413 fpm) air velocity: Standard Australian conditions (Australian Standard AS 2913-2000) [3]. Standard American conditions (CEC Title 20) [5]. Typical temperate European summer conditions. Table 1 Comparison under Australian conditions Quantity Standalone IntrCooll Inlet dry bulb 38.0 C 38.0 C temperature (100.4 F) (100.4 F) The dark blue arrow represents the pre-cooling process. Light blue arrows represent air passing through the lower half of the Oxyvap pad. Red arrows represent air passing through the upper half of the Oxyvap pad. 3.2 Variations There are two main variations on the IntrCooll cooling configuration: The "standard" configuration in which the air-towater heat exchanger has the same width and height as the Oxyvap pad. All inlet air is being pre-cooled prior to its passage through the pad. Inlet wet bulb temperature 21.0 C (69.8 F) 21.0 C (69.8 F) Room dry bulb temperature 27.4 C (81.3 F) 27.4 C (81.3 F) Supply temperature* 23.0 C (73.4 F) 18.7 C (65.7 F) Supply absolute humidity* 14.7 g/kg (0.0147 lb/lb) 11.5 g/kg (0.0115 lb/lb) et bulb efficiency 88% 113% Cooling capacity The "compact" configuration in which the air-towater heat exchanger has the same width as the Oxyvap pad, but only half the height, hereby precooling only the lower 50% of the inlet air. 1.5 m3 /h 3.0 m3 /h per airflow (8.7 (17.2 *Average of the lower 50% of air leaving the pad. The standard configuration has the better cooling performance, whereas the compact configuration is favorable because of the lower initial costs. Simulations have shown that an air-to-water heat exchanger of two-thirds of the height of the Oxyvap pad is the ideal trade-off between performance and costs. This is the configuration shown in figure 3. 3
Table 2 Comparison under American conditions Quantity Standalone IntrCooll Inlet dry bulb 32.8 C 32.8 C temperature (91.0 F) (91.0 F) Inlet wet bulb 20.6 C 20.6 C temperature (69.0 F) (69.0 F) Room dry bulb 26.7 C 26.7 C temperature (80.0 F) (80.0 F) Supply 22.1 C 19.1 C temperature* (71.8 F) (66.4 F) Supply abolute 14.6 g/kg 12.3 g/kg humidity* (0.0146 lb/lb) (0.0123 lb/lb) et bulb efficiency 88% 112% Cooling capacity 1.6 2.6 per airflow (9.1 (15.0 *Average of the lower 50% of air leaving the pad. Table 3 Comparison under European conditions Quantity Standalone IntrCooll Inlet dry bulb 28.0 C 28.0 C temperature (82.4 F) (82.4 F) Inlet relative humidity 50% 50% Room dry bulb 27.3 C 27.3 C temperature (81.1 F) (81.1 F) Advantages of the IntrCooll compared to standalone direct evaporative cooling: Capable of cooling to well below the initial wet bulb temperature. Significant increase in cooling capacity. Less increase in absolute humidity. Larger applicability because the technology is able to function well throughout a wider range of outdoor air conditions. 4 PreCooll 4.1 General principle A conventional vapor compression air conditioning system usually consists of an air cooled condenser placed outside. A compressor is used to compress the gaseous coolant before it will lose its heat by condensation in the condenser coil. This heat is transported to the outdoor environment predominantly by means of forced convection. To accomplish this, the condenser should be brought to a significantly higher temperature than its surroundings. The work that has to be done by the compressor is dependent on the required condensing temperature, that is proportional to the condenser inlet temperature, i.e. the outdoor air temperature. Should this temperature be reduced, so would the condensing temperature. Figure 5 Condenser principle (airside) Supply 21.2 C 19.5 C temperature* (70.2 F) (67.1 F) Supply absolute 14.5 g/kg 13.1 g/kg humidity* (0.0145 lb/lb) (0.0131 lb/lb) et bulb efficiency 88% 110% Cooling capacity 2.1 2.7 per airflow (12.1 (15.4 *Average of the lower 50% of air leaving the pad. 4
Copeland ZRT216KCE-TFD: The Oxyvap direct evaporating cooling pad can be used to pre-cool the outdoor air that is supplied to the condenser coil, hereby decreasing the work that has to be done by the compressor and consequently increasing the efficiency of the system. To this end, Oxycom developed the PreCooll system. Source: Copeland Selection Software Version 7.10 / 41936 (10/14). Manufacturer: Emerson Climate Technologies. Type: Scroll compressor. Figure 6 PreCooll schematics Power supply: 380/420 V, 3ph, 50 Hz. Refrigerant: R407C, R134a. Evaporating temperature: 0 C (32 F), 5 C (41 F), 10 C (50 F). Superheat: 10 K (18 R). Condensing temperature: 25 60 C (77 140 F) for R407C, 25 70 C (77 158 F) for R134a. Subcooling: 0 K (0 R). 1. Hot outdoor air. 2. Cold and humid condenser inlet air. Figure 7 ZRT216KCE-TFD COP values 3. Hot and humid exhaust air. 4.2 Efficiency There are two quantities in which the compressor efficiency can be expressed: Coefficient of Performance (COP). Energy Efficiency Ratio (EER). Although some manufactures differentiate between EER for cooling and COP for heating, this paper adopts the ASHRAE definition [4], in which both numbers refer to the cooling operation of a vapor compression air condi- Figure 8 ZRT216KCE-TFD COP increase tioning system, but expressing COP in SI units (k/k) and EER in I-P units (BTU/h). 4.3 Impact Two randomly selected compressors from different manufacturers have been analysed to illustrate the impact of the condensation temperature on the efficiency. 5
Danfoss MTZ080-4: Advantages of the PreCooll system applied to conventional vapor compression air conditioning technology: Source: Danfoss Foresee V3.2.4 - Data V5.7.0 DFS_Calc V3.2.0 - ASEREP, Version 3.5.0. Analysis of the selected compressors and refrigerants show a COP increase of 2.5 5.0% per kelvin decrease in condensing temperature (1.4 2.8% per degree Rankine). The average value is 3.5 %/K (1.9 %/ R). Manufacturer: Danfoss. Type: Reciprocating compressor. Power supply: 400 V, 3ph, 50 Hz. For example, 10 K (18 R) pre-cooling of the outdoor air allows for an equal decrease of the condensing temperature, leading to 41% average increase in compressor efficiency, corresponding to 29% reduction in energy consumption. Refrigerant: R407C, R134a. Evaporating temperature: 0 C (32 F), 5 C (41 F), 10 C (50 F). Superheat: 10 K (18 R). Reduction of the condensing temperature not only increases the compressor efficiency, it also increases the cooling capacity. hen a compressor is equipped with modulating technology, it can reduce the cooling capacity (i.e. operating in part-load) to increase its efficiency even further. Condensing temperature: 35 65 C (95 149 F). Subcooling: 0 K (0 R). Figure 9 MTZ080-4 COP values References [1] Oxycom FRESH AIR BV, Evaporative Cooling Principles, XRD-Z-020-1 (2013). [2] TNO, Efficiency test of a prototype Oxyvap direct evaporative cooling pad, 060-APD-2012-00118 / TNO2012 M10316 (2012). [3] Standards Australia, Australian StandardTM, Evaporative airconditioning equipment, AS 2913-2000 (2000). [4] American Society of Heating, Refrigerating and AirConditioning Engineers, 2009 ASHRAE HandbookFundamentals (SI) (2009). Figure 10 MTZ080-4 COP increase [5] California Energy Commission, 2012 Appliance Efficiency Regulations, CEC-400-2012-019-CMF (2012). 6