Keywords

8.1 Introduction

Determining the actual performance of an installed heating, ventilating, and air-conditioning system (HVAC) system is very important for determining its performance, behavior, and any possible problems [1]. Applying the appropriate thermal capacity system based on the thermal load of buildings makes the system operate efficiently [2, 3]. Hence, determining the system’s thermal capacity is important to understanding if the installed system is performing as expected in terms of its design and installation. As variable refrigerant flow (VRF) systems are becoming popular since they can be operated variably, installed performance evaluations are important to keeping their operating costs low and ensuring that maintenance can be done quickly.

The measurement of an HVAC system’s thermal capacity can be done using enthalpy methods, either in air or refrigerant. The problem with the refrigerant enthalpy method is that the measurement of refrigerant flow rate is not simple in the two-phase state, as mentioned in some articles on two-phase flow measurements [4,5,6]. Hence, during the dynamic operation of a VRF system, measurements of refrigerant mass flow rate in different indoor units are more difficult, as unsteady two-phase flow is expected to occur.

Another method for calculating thermal capacity is to use the air enthalpy method [7]. The calculation of air enthalpy is based on the moist air temperature and humidity. In addition, the measurement of the moist air mass flow rate based on variable air fan speed operation is needed. If the above parameters are known, the system’s thermal capacity can be accurately calculated, assuming that the sensors and instruments used to measure the primary variables are reliable and accurate.

As VRF systems are expected to operate variably, depending on indoor and outdoor conditions, it is important to accurately measure the primary variables—namely, air temperature, air humidity, and air mass flow rates. Hence, it is important to determine the methods, instruments, and sensors to use when measuring the above variables based on the actual installed conditions of a VRF system in its real environment.

This chapter presents a method for measuring the thermal capacity of a VRF system. The authors used this method to evaluate VRF system behavior, performance, and operation [8,9,10]. The presented information is based on the authors’ actual experiences related to the installation and application of sensors and instruments to measure the air temperature and humidity, as well as the measurement of air flow rates based on the variable air fan speed of VRF systems in indoor units.

8.2 Methodology

8.2.1 Test Facility

The artificial control chambers testing facility is used for the thermal capacity evaluation of the VRF system. The VRF system is installed in the testing facility in which the thermal load, air temperature, and air humidity can be varied, both for outdoor and indoor conditions. Figure 8.1 shows the testing facility where the VRF system was installed. Figure 8.1a shows the actual view of the facility, and Fig. 8.1b shows the schematic diagram of both the outdoor and indoor test chambers. The facility consists of one outdoor chamber and three indoor chambers. Indoor chambers 1 and 2 are separated. The two indoor units can be installed in tandem in indoor chamber 3. In this study, indoor chambers 1 and 2 are used. Monitoring and control devices using different sensors are installed to change, monitor, and store the independent parameters using computers.

Fig. 8.1
Two photos and a schematic of a test facility. a) inside and outside view photos. b) schematic of 3 indoor chambers, an outdoor chamber with an outdoor unit, interconnected by a refrigerant pipe.

Test facility for the evaluation of the VRF system: a Actual view showing the test chambers (both outside and inside) and b a schematic diagram of the test facility

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8.2.2 Test Specimen

A commercial VRF system is used as the test specimen for this research. (Fig. 8.2). Figure 8.2a shows the outdoor chamber with the outdoor unit of the VRF system under testing. Figure 8.2b is a schematic diagram of the VRF system showing all the installed sensors. Figure 8.3 shows a VRF system indoor unit (IU) installed in the indoor chamber of the test facility. Air temperature and relative humidity sensors are installed in the indoor unit’s return air and supply air. One unit of temperature and relative humidity sensor is used in the return air part, and four temperature and relative humidity sensors are installed in the supply air part, as it has four supply air parts (Fig. 8.3a). The most accurate temperature and relative humidity sensors available for a larger range were installed to obtain the most accurate readings possible. A laser rotational sensor is installed in the shaft of the air fan in all the indoor units. Figure 8.3b shows the schematic diagram of the indoor units, where the return air is processed before it becomes supply air. The diagram shows the air fan, air vane, and the location of the attached sensors.

Fig. 8.2
A photo and a schematic. a) photo of the VRF system. b) An outdoor chamber and 2 indoor chambers interconnected, each has an electric motor and sensors for pressure, temperature, flow etcetera.

VRF system under testing and evaluation: a View of the outdoor chamber with the outdoor unit under testing and b a schematic diagram of the VRF system under testing and evaluation

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Fig. 8.3
A photo, a) and a schematic b), of the indoor unit with supply and return air thermocouple wire, temperature, and relative humidity sensors, and laser rotation sensors marked.

VRF system indoor unit (IU): a Actual view of the IU with attached sensors and b a schematic diagram of the IU. T–dry bulb temperature, RH–relative humidity

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8.2.3 Test Method

When the VRF system is running, all the sensors shown in Fig. 8.2 gather all the data every five seconds. Moreover, the rotational speed of the indoor units versus the air flow rates at different fan settings and air vane settings are gathered using the specialized air flow measurement device shown in Fig. 8.4. Figure 8.4a shows the air flow measurement device being prepared in one of the indoor units for air flow measurements. Figure 8.4b shows the schematic diagram of the air flow measurement device. This device eliminates the pressure loss created by the orifice and ducting by the pressure loss compensation fan, which is done automatically by the device to balance the pressure between the outside air and the return air. The air flow is measured using a digital orifice flowmeter with memory. In the case of this VRF system test specimen, air flows based on different fan speeds and van directions were measured based on three different air fan speeds (low, medium, and high). All the precautionary measures have been applied and referenced based on existing testing and measurement standards [11].

Fig. 8.4
A photo (a) and schematic (b) of the air flow measurement device connected to the indoor unit. An office flow meter and pressure loss compensation fan are within the connecting pipe.

Air flow measurement device for the measurement of actual air flow rate in the indoor units (IUs): a Preparation of air flow measurement using the device and b a schematic diagram of the air flow measurement device

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8.2.4 Test Evaluation

The air flows for different speeds and vane directions are measured to obtain the air enthalpy and the thermal capacity of the installed VRF system. The fan speed was measured using the laser rotation sensor attached to the fan shafting. The air temperature and humidity, both for return air and supply air (See Fig. 8.2b), are measured using the installed air temperature and humidity sensors. One air temperature and humidity sensor is attached to the return air location, and four air temperature and humidity sensors are attached to the supply air location, as it has four supply outlets (Fig. 8.1b). The calculation of the air enthalpy and mass air flow rate is based on moist air.

The enthalpy of moist air is expressed as:

$$ h = h_{a} + wh_{g} $$
(8.1)

where

h :

total air enthalpy

h a :

dry air enthalpy

h g :

water vapor enthalpy

w :

humidity factor.

The thermal capacity is expressed as:

$$ Q_{C} = \left| {\mathop \sum \limits_{1}^{n} \left( {h_{r,n} - h_{s,n} } \right)} \right| $$
(8.2)

where

Q c :

thermal capacity

m a :

mass flow of moist air

h r :

moist air enthalpy of return air

h s :

moist air enthalpy of supply air

n :

number of indoor units.

8.3 Results and Discussion

8.3.1 Air Fan Rotation and Flow Rate Correlation

Figure 8.5 shows the sample correlation gathered after the air fan speed and air flow rates were measured for different air vane angles using the air flow measurement device. This kind of correlation was gathered for all different VRF systems to be tested in the controlled chambers for actual performance evaluation. Figures 8.4a, b show the correlation for the horizontal air vane position, Figs. 8.4c, d show the correlation when the air vane was in an inclined position, and Figs. 8.4e, f show the correlation for the vertical air vane position. With the gathered air flow and fan rotation correlation, air flow can be measured accurately for the performance evaluation of the installed VRF system.

Fig. 8.5
Six graphs of air flow rate versus fan speed. Plotted at horizontal vane a) unit 1, b) unit 2, 45-degree vane c) unit 1, d) unit 2. At downward vane, e) unit 1 f) unit 2. The curves linearly increase.

Fan speed and air flow correlations for different fan speeds and air vane directions: a–b horizontal position, c–d inclined position, and e–f vertical position

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8.3.2 Indoor Units Air Fan

Figure 8.6 shows the actual air fan speed measured using the laser rotational sensors when the installed VRF system was running under actual operational conditions. The presented sample results are based on the steady operation of the VRF system in which the rated capacity is measured based on actual installed conditions. Based on the results, the fan air speed remained almost constant during this time. Also, during the dynamic operation of the VRF system, an on–off fan speed or varying fan speed could be measured, which can be translated to air flow rates based on the correlation developed for the VRF system during the performance evaluation.

Fig. 8.6
A graph of fan speed versus time for indoor units 1 and 2. The fan speed remains constant at about 3o rotations per second in both units.

Air fan speed measurements of actual indoor units (IUs)

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Figure 8.7 shows the air temperature and humidity ratio of the supply air and return air. The installed air temperature and relative humidity sensors to be installed in the measurement of the VRF system should be as accurate as possible. The sensors with the highest accuracy are available but cost more than other sensors. However, with the performance evaluation of the VRF system, expensive sensors and measurement instruments are important to get reliable results.

Fig. 8.7
Two graphs of dry bulb temperature and humidity ratio versus time. Plotted for supply air and return air at a) unit 1 and b) unit 2. All values are seen to be almost constant throughout.

Air temperature and moisture content of supply air (SA) and return air (RA): a Indoor unit 1 (IU1) and b Indoor unit 2 (IU2)

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8.3.3 Air Flow and Air Enthalpy

Figure 8.8 shows the calculated air flow rates and air enthalpy of the air flowing in the indoor units. All the calculations were based on moist air. The moist air enthalpy and moist air mass flow were calculated to obtain more accurate results for the flowing air. Based on the results, during the stable VRF operation, the air flow rate and air enthalpy remained stable, as expected. During the performance evaluation of the VRF system, these parameters were constantly monitored to check the behavior of the VRF system and the controlled chambers, as well as the possible effect of outdoor conditions.

Fig. 8.8
Two graphs of air enthalpy of supply air and return air, and air flow. Plotted versus time at a) unit 1 and b) unit 2. The 3 curves in both units remain almost constant throughout.

Air enthalpy of the supply air (SA), return air (RA), and air flow rate: a Indoor unit 1 (IU1) and b Indoor unit 2 (IU2)

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8.3.4 Thermal Capacity

Figure 8.9 shows the calculated VRF system thermal capacity (which is a product of the difference in air enthalpy between the return and supply air and the air flow rate). As presented in the results, the calculated thermal capacity was almost the same as the simulated thermal load. The thermal load was the heat load artificially produced by the indoor chambers, which the VRF system absorbed during cooling operation mode. During heating operation, the VRF system produced heat as the indoor chambers’ air handling units absorbed the heating capacity produced by the VRF system. Based on these results, the air enthalpy method can be used to evaluate the performance of the VRF system, as it can be almost the same as the thermal load of the indoor chambers.

Fig. 8.9
A graph of Thermal loading and VRF thermal capacity against time. Also, their percentage difference is plotted. Both curves are constant and similar throughout, so the difference curve is near zero.

Indoor chambers’ thermal load and indoor units’ (IUs’) thermal capacity

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8.4 Conclusions

This chapter presented the air enthalpy method for measuring a VRF system’s thermal capacity. It showed the conducted measurements of air flow, fan speed rotation, air temperature, and humidity. This method was used to evaluate the VRF system’s thermal capacity and performance, as presented by Enteria et al. [8,9,10].

The specific conclusions that can be drawn from this study are as follows:

It is important to measure air flow, temperature, and humidity, both in the return air and supply air of the VRF’s IUs, using high accuracy sensors.

It is important to measure air flow rates based on fan rotational speed to obtain the correlation formulation to be used to measure the air flow of the installed VRF system.

Based on the VRF thermal capacity calculated using the air enthalpy method, the measured capacity is almost the same as the simulated thermal load of the controlled chambers.

These conclusions indicate that the air enthalpy method is reliable for evaluating the installed VRF system’s thermal capacity. As such, measurements of installed VRF system thermal capacity in actual buildings obtained using air enthalpy are reliable if the sensors, instrumentation, and measurements of air flow rates are selective and done properly.