Technical Report An Introduction of Below-Freezing and Humidity Environment Controlling Technology Equipped on Brake Test Equipment Hironobu Kurara, Custom Equipment Engineering Department 2, Custom Equipment Headquarters, ESPEC CORP. Abstract Various types of brake tests are being performed in the automotive market using a brake dynamometer. ESPEC has previously been engaged in the creation of temperature and humidity environment for brake testing equipment for performing these tests, but in recent years, there has been an increase in the demand for below-freezing temperature and humidity environment control. This report is an introduction of the below-freezing temperature and humidity environment control technology (patented) used on brake testing equipment for ESPEC to achieve this requirement. Keywords Brake testing, brake dynamo, below-freezing temperature and humidity control, automotive market 1. A below-freezing temperature and humidity environment Below 0 C (below-freezing), water exists in the form of ice or supercooled water. Therefore, the relative humidity below-freezing consists of both the relative humidity related to the saturated vapor pressure from the supercooled water and the relative humidity related to the saturated vapor pressure of the ice. Fig. 1 shows a graph of the saturated vapor pressure of the water and ice. Although they do not differ greatly, the saturated vapor pressure of the ice is a lower value. Commonly, ESPEC defines relative humidity by the saturated vapor pressure of supercooled water. (In conformance with Japanese Industrial Standards (JIS) B7920 Hygrometer - test method as well as Z8806 Humidity - measurement methods.) Also, when below-freezing, there is a physical property related to Dew point temperature called Frost point temperature. Related to where the temperature of saturated vapor pressure of water is the same as the partial pressure of water vapor at that point being the dew point pressure, frost point temperature is where the temperature of saturated vapor pressure of ice is the same as the partia l pressure of water at that point. Fig. 2 shows a graph of the frost point temperature and the dew point temperature of water during saturation vaporization. The frost point temperature is higher than the dew point temperature. In summary, when cooling an object, the surface temperature of the object when frost begins to form on the surface is the frost point temperature, and the temperature of the object when dew begins to form due to saturation from a supercooled water state is the dew point temperature. When controlling the humidity where dew point temperature is lower than frost point temperature, there are cases where ESPEC will define relative humidity based on the saturated vapor pressure of ice. Test Navi Report No. 25 (98th Issue) 2017 1
Fig. 1 Below-freezing saturated vapor pressure Fig. 2 Dew point temperature and frost point temperature at saturated vapor pressure of water 2. Issues when performing below-freezing temperature and humidity environment control using a typical air conditioning system At ESPEC we use one of our typical air conditioning systems to produce a temperature and humidity environment for brake testing device. Temperature and humidity control is performed using output controllers inside of the various air conditioning devices which are arranged in an air conditioning process; for heating, an electric heater, for humidification, a vapor humidifier, for dehumidification and cooling a plate fin heat exchanger. When creating a below-freezing humid environment using this system, there are two main issues. (1) Frost on indoor objects/components Commonly, due to the difference in water vapor pressure, the saturated vapor based on vapor humidification will diffuse into conditioned air and will change in temperature, but in a below-freezing environment, it will transition into a supercooled state, and as it remains in the atmosphere as moisture content, a large amount of the saturated vapor will form into contact ice and frost on objects with a temperature under the frost point faster than its diffusion or the rate of temperature change. Due to this frosting, there is a decrease in moisture in the air, creating a possibility that the intended testing environment cannot be realized. (2) Frost on plate fin heat exchanger Fig. 3 shows a plate fin heat exchanger that is a dehumidifying cooler. Composed of cooling tubes that circulate coolant directly and heat exchange fins that cool conditioned air with dehumidification, of which both use copper as a material that has an extremely high thermal conductivity. Fig. 4 shows the analysis results of the surface temperature distribution of the heat exchange fins on the coolant evaporation part during below-freezing air conditioning operation. For analysis conditions, roughly the same kind of coolant is circulated in the cooling tubes as in actual operating conditions, coolant being (HFC-R404A) temperature -40 C, and air conditioning temperature being -20 C. In these results, the fin surface was between -38.9 C and -40 C, not being affected by conditioned air, approximately the same as coolant temperature. Moisture content that is dehumidified from below-freezing air will form as frost in the air conditioning system on the surface of the dehumidifying cooler surface, but as the Test Navi Report No. 25 (98th Issue) 2017 2
surface temperature of the cooling fins is roughly uniform, frost will form evenly on the entire radiator. If moisture content decreases due to frost from air conditioning, the quantity of frost will gradually increase in thickness if vapor humidification is continued, and in a short time, frost conditions such as in Fig. 5 will exist, making air conditioning control impossible. Heat exchange fine (Copper) Minimum: 233.150 Maximum: 234.220 Air: -20.0 C Thermal conductivity 20W/m2 C Cooling tubes (Copper pipe) Fig. 3 Dehumidifying cooler (plate fin heat exchanger) Air temperature -20.0 Av erage temperature -39.0 Coolant temperature -40.0 ΔT (Coolant) 1.0 Fig. 4 Heat exchanging fin surface temperature simulation Fig. 5 Frost conditions on plate fin heat exchanger 3. Considerations for below-freezing temperature environment control If humidity is not added to the air, and only the moisture content in the air all forms as frost on the dehumidifying radiator, to what degree would frost form? Table 1 shows a comparison of below-freezing high humidity air conditions (-10 C 80%rh) and standard air condition (+23 C 50%rh). The absolute humidity of standard air (+23 C 50%rh) is approximately 8.75g / kg (DA), and when considering ESPEC s standard environment testing room 3 (approximately 12.5 m3), there is only approximately 126 g of moisture in the room. In below-freezing high humidity air conditions (-10 C 80%rh), it is lower, with only 22.9 g of moisture. For example, even if the 126 g of moisture in standard air conditions formed as frost with a density of 0.3 g/cm3 on the surface of ESPEC s dehumidifying radiator in our environment testing room, with a uniform adhesion, the thickness of that frost amounts to a mere 0.02 mm, and is an amount that will likely have almost no effect on the heat exchange of the air conditioner. Test Navi Report No. 25 (98th Issue) 2017 3
Calling attention to this and the aforementioned fact that moisture content that is dehumidified below-freezing will form as frost on the dehumidifying cooler (within the air conditioning system), we have derived that by controlling the quantity of frost formation on dehumidifying coolers in a below-freezing environment will allow for the control of the amount of moisture content in conditioned air. That is to say, humidity control is possible. These are our considerations. 4. Overview of the ESPEC s below-freezing humidity control system In air conditioning, if the surface temperature of the dehumidifying cooler becomes equal to that of dew point of conditioned air, then the moisture content removal due to the sublimation of the frost on the surface of the dehumidifying radiator will enter an equilibrium state, creating constant air conditioned humidity. Consequently, to control the amount of frost formed, it is necessary to control the dehumidifying cooler surface temperature. As long as the coolant flowing in the coolant tubes of the dehumidifying radiator does not evaporate causing an overheating condition, temperature and pressure are roughly at constant. Fig. 6 shows a PT diagram of the coolant generally used by ESPEC for air conditioning HFC-R404A (pressure - temperature diagram) Fig. 7 is a figure showing the frost point temperature and the dew point temperature below an ambient temperature of -10 C. From these two diagrams, we show that in order to have control at -10 C 80%rh, it is necessary to set the surface temperature of the dehumidifying cooler to the temperature and humidity dew point temperature (-12.8 C) and frost point temperature (-11.4 C). To do this, we need to regulate the coolant pressure in the dehumidifying cooler at around 0.30 MPaG. Fig. 6 Saturation state of HFC-404A Fig. 7 shows the frost point temperature and the dew point temperature at an ambient temperature of -10 C. Test Navi Report No. 25 (98th Issue) 2017 4
Fig. 8 is an overview of the below-freezing humidity control system. The main control equipment used to produce a constant below-freezing temperature and humidity are shown in the figure: EPV (Electronic coolant pressure control valve), ELV (Electronic coolant flow control valve), and AMV (Motor valve). These three equipment are as mentioned below. The EPV adjusts the internal pressure of the dehumidifying cooler by opening and closing. When the valve is opened, the internal pressure of the dehumidifying cooler will decrease, decreasing the humidity in the test chamber. The opposite effect is seen when closed. Opening and closing of the ELV varies the internal coolant flow in the dehumidifying cooler and this adjusts evaporation temperature. Larger area for vaporizing coolant leads to more uniform surface temperature of the dehumidifying cooler, improving the response of humidity control using pressure adjustment, thus becoming more stable. The output and input temperatures of the dehumidifying cooler are utilized for this opening adjustment. Lastly, the AMV opens to introduce outside air into the air conditioning device. As the below-freezing humidity control area definitely has lower absolute moisture content than outside air, introducing outside air humidifies the air conditioned room. This is used for times when operation is started to form a proper amount of frost on the dehumidifying cooler, or during long periods of operation where there is insufficient moisture content in the air conditioning system. Fig. 9 shows a chart graph of the system introduced above conducting a below-freezing temperature and humidity operation. This shows that temperature and humidity can be controlled with extreme stability. Controller Test chamber T Open air H Coolant E TS M AMV Air conditioner EVA PS TS E ELV EPV EVA: Cooling dehumidifier PS: Pressure sensor T: Temperature sensor for air conditioner EPV: Electronic coolant pressure control valve TS: Temperature sensor H: Humidity sensor for air conditioner ELV: Electronic coolant flow control valve AMV: Motor valve [for air] Fig. 8 Below-freezing humidity control system diagram Test Navi Report No. 25 (98th Issue) 2017 5
Fig. 9 Below-freezing temperature and humidity operation chart graph 5. Development of below-freezing humidity control technology At this present time, a product equipped with this technology is the Walk-in Type Temperature & Humidity Chamber, E Series. Also, the controllable range of below-freezing humidity is shown in Fig. 10. The solid line shows standard specification, and the dotted line shows the confirmed controllable range of below-freezing temperature and humidity control. Currently, the range in which temperature and humidity can be specified is limited, but through system improvement and addition of functions, the confirmed controllable range is expanding. The greatest feature of this control system is that by controlling the amount of frost formation, consecutive operation for long periods of time is possible, regardless of temperature and humidity operations in below-freezing areas. Currently, this technology is only used for brake testing device, but we are considering leveraging these features for development in other fields. Test Navi Report No. 25 (98th Issue) 2017 6
Fig. 10 Range humidity control can be specified while below freezing Fig. 11 Walk-in Type Temperature (& Humidity) Chamber, E Series Test Navi Report No. 25 (98th Issue) 2017 7