Effect of Inlet Duct Design on Fan Performance of Indoor Air Handling Units with Pull-Through Fan Configuration (ASHRAE RP-1743)

During testing indoor air handling units, ASHRAE Standard 37 requires installing an inlet duct that might increase the length of the testing setup to exceed the capability of some testing facilities. ASHRAE Research Project 1743 (ASHRAE RP1743) addresses this issue by evaluating the effect of inlet duct design on fan performance of indoor air handling units. The findings of the study will allow identifying alternative, shorter, inlet duct configurations maintaining the fan performance as of that obtained with ASHRAE Standard 37 inlet duct. The test matrix of the project includes various indoor units differing in motor type, such as Constant Torque Motor (CTM) and Electronically Commutated Motor (ECM), and fan configurations, such as pullthrough and pushthrough configuration. This paper focuses on the findings of testing two pullthroughfanconfiguration units, one of which with a CTM fan and the other with an ECM fan. In addition, other effects, such as atmospheric pressure and hysteresis in approaching external static pressure (ESP), were investigated to study their impacts on the repeatability of equipment testing. The results of the CTMfan unit showed that the proposed alternative inlet duct configurations can maintain the fan performance within the suggested tolerances, which are 5% in power and 2.5% in air volumetric flowrate relative to the ASHRAE Standard 37 inlet duct. This allows a reduction in the inlet duct stacklength from 38% to 74% compared to ASHRAE Standard 37 inlet duct configuration. Noticeable performance hysteresis was observed at low air flowrate and ESP, while atmospheric pressure had no effect on fan performance. For the ECMfan unit, the fan performance was maintained during testing the proposed alternative inlet duct configurations within the suggested tolerance, with a noticeable decrease in power during testing the reduced alternative inlet duct configuration. The reduction in the stack length as compared to the standard inlet duct configuration was 40%, 72%, 74% for the inlet box, 4inch inlet duct, and reduced alternative configuration, respectively. In contrast to the results of the CTMfan unit, the hysteresis effect on the ECMfan unit was found to be insignificant, while atmospheric pressure effect was significant. As the atmospheric pressure increased, air volumetric flowrate and fan’s power decreased

Dynamic Modeling of Packaged Air Conditioner with Micro-Channel Heat Exchanger Condenser

Microchannel Heat Exchangers (MCHX) are used in Air Conditioning systems (AC) as an efficient type of Heat Exchangers (HX) because they have compact size, as well as low refrigerant charge. However, using MCHX as condensers can be associated with some problems, including evaporatorcondenser transient charge balance issues. Therefore, this study aims to investigate the impact of using MCHX as a condenser in a packaged air conditioner with a finandtube heat exchanger as an evaporator. The methodology was dynamically modeling the packaged air conditioner in Modelica, considering varying pass configurations of MCHX with several refrigerant charges. The results showed that the pressure in the highpressure side of the refrigerant cycle increases as the number of tubes in the inletpass of the MCHX decreases. In addition, if the difference in the number of tubes among the inletpass and the outletpass is large, the pressure increases regardless of which pass has the highest number of tubes. Moreover, the pressure can increase after a while at the beginning of an oncycle due to the slow response of Thermostatic Expansion Valve (TXV) caused by the thermal inertia of the sensing bulb. We defined a mass ratio as mass of refrigerant in the evaporator to the mass in the condenser. A decrease in the mass ratio for a given total charge corresponds to the refrigerant accumulating in the condenser, increasing pressure in the condenser

Feasibility of Controlling Heat and Enthalpy Wheel Effectiveness to Achieve Optimal Closed DOAS Operation

Dedicated outdoor air systems (DOAS) make use of the sensible and latent capacity of the return (exhaust) air to cool and dehumidify the supply (outside) air. “Closed DAOS” is an advanced form of typical DOAS with desuperheater and evaporatively cooled condenser in the return air stream. The air entering the condenser in Closed DOAS is in the fully saturated state. In order to fully saturate the return air, it needs to be sprayed by water. Sufficient amounts of water is required for fully saturating the air and keeping the coils of the condenser wet during the process. This water is obtained internally as a result of condensation on the evaporator fins, not requiring additional water use at the buildings site. Closed packaged configuration requires to have a water balance of water obtained in evaporator to that required for saturating the air and wetting the condenser coil. This water balance can be achieved under most conditions by an optimal control of enthalpy wheel and heat wheel (run around heat exchanger) effectiveness to control the amount of water obtained in the evaporator. The optimal control of heat and enthalpy wheel effectiveness is not only important for making the necessary water balance but also to keep the latent load as low as possible. This paper discusses the Closed DAOS configuration with psychrometrics and water balance analyses

A Literature Review of Numerical Modeling Techniques for Vapor Compression Systems with Focus on Heat Exchanger Modeling

Vapor compression systems are the most widely used system type in heating, ventilation, air-conditioning, and refrigeration (HVAC&R) applications. Experimental and numerical modeling techniques are used to analyze the performance of the vapor compression systems. With the introduction of high-performance computers, numerical modeling techniques are used extensively to develop cost-effective and efficient HVAC&R equipment. Experimental iterations on the design of vapor compression systems are costly; however, numerical techniques can reduce the number of experimental iterations, substantially decreasing the development cost and time. Because of the benefits associated with the numerical simulation, many researchers working in the HVAC&R field have attempted to develop efficient, robust, and accurate simulation models. This paper provides an in-depth review of heat exchanger modeling techniques as well as integration strategies to develop holistic system models

Effect of Inlet Duct and Damper Design on Fan Performance and Static Pressure Measurements (ASHRAE RP 1743)

ASHARE and AHRI provides various standards for designing a test setup for HVAC systems, but none of those specify sufficient details for the inlet ductwork design. The purpose of this study is to develop an inlet duct design guideline for inclusion in the AHRI and ASHRAE testing standards which may reduce the risk of false testing failures and will lead to higher integrity of the testing results at different laboratories. This study will evaluate the performance of the fan of the indoor air handler of split systems for various design parameters including the fan and motor type of the air handling unit, inlet duct and damper configurations and air flow rate through the unit. Fan power consumption and air flowrate of the system are to be determined for a range of static pressures for different inlet duct/damper configurations. The test plan includes indoor air handlers of 1.5-ton, 3-ton and 5-ton capacity. For the air handling unit’s three types of motors and two types of fans are considered. Static pressures will be measured between the inlet and outlet of the air handler.Finally, the experimental results will be used to validate CFD simulations of the inlet ductwork

CFD Case Study: Heat Exchanger Inlet Air Velocity Distribution for Ducted Tests in a Psychrometric Chamber (ASHRAE RP-1785)

This paper presents the airside analysis of coil duct configurations using Computational Fluid Dynamics (CFD) to determine which configuration best mitigates airside maldistribution for ASHRAE RP-1785. RP-1785 has a global objective of providing accurate refrigerant charge and oil retention data for residential coils, collected in a controlled experiment. The final test matrix of the study includes several representative residential indoor and outdoor coils to be tested at various refrigerant and airside inlet conditions. The coil performance, charge, and oil retention behavior is strongly influenced by the airflow; therefore the uniform airflow distribution to coils is critical to RP-1785 to maintain a well-controlled experiment. Analysis of four 3D CFD cases are presented with the largest coil of the initial test matrix (105.6 in (268.2 cm) length, 40 in (101.6 cm) height, 5 in (12.7 cm) depth) installed in the Oklahoma State University psychrometric chambers. The simulation domain was extended to include the airflow characteristics within the psychrometric chamber to determine the effect of the asymmetric air inlet boundary condition. The analysis concluded the three significant factors affecting the airflow uniformity: the distance between the duct and wall, the distance between the duct and chamber floor, and the upward incoming airflow area of the floor. The CFD study results are used to inform the design of the duct to be used for coil testing in RP-1785 and the final duct design is presented

A Virtual EXV Mass Flow Sensor for Applications With Two-Phase Flow Inlet Conditions

In conventional vapor compression systems, electronic expansion valves (EXVs) are used for refrigerant flow control. Subcooled refrigerant enters the expansion device and is expanded to the evaporation pressure while the valve opening is modified to achieve the desired mass flowrate. The relationship between the inlet and outlet conditions, the opening, and the mass flowrate has been extensively studied, e.g. by Park et al. (2007) and appropriate empirical correlations have been developed. However, for certain operating conditions (e.g. low refrigerant charge) or applications that generally have two-phase inlet conditions (e.g. balancing valves used in a hybrid control scheme as proposed by Kim et al. (2008)), these correlations are not applicable, since even low inlet vapor fractions lead to a significant reduction of the valve mass flowrate at a given opening. This paper proposes a continuous correlation that can be used for both two-phase and subcooled valve inlet conditions. The benefit of the continuity is that there is a smooth transition between subcooled and two-phase inlet conditions, which is essential for control and simulation purposes. The new correlation employs the Buckingham-Pi theorem as proposed by Buckingham (1914). The selected dimensionless Pi-groups describe opening of the valve, subcooling, inlet and outlet pressures, driving pressure difference across the valve, inlet density, surface tension, and viscosity. The data that was used to determine the coefficients of the correlation was taken on a dedicated valve test stand, which was sized for the per-circuit capacity of a typical 5-ton R410A heat pump and a 3-ton R404A large room cooling system. The purpose of these tests was mainly to map the valves for the low pressure drops, high inlet qualities and large valve openings that occur when they are used as balancing valves in a hybrid control approach. Two commercially available valves of different rated capacity were tested. Due to the much higher valve capacity for subcooled inlet conditions, valve openings of less than 5% occurred in that case. This led to an accuracy of the correlation for these points that is less than what typically can be found for correlations with subcooled inlet conditions in the open literature. However, for two-phase flow inlet conditions, the resulting RMS of 1.0 g/s for the 8-PI correlation is sufficiently small to use the approach for estimating the refrigerant mass flow and using the EXV as a virtual flow sensor. The limitations of this approach in practical applications, as well as possible applications in fault detection and diagnostics are shown for application as balancing valves within a 5-ton R410A heat pump and a 3-ton R404A large room cooling system