HVAC SYSTEM REMOTE MONITORING AND DIAGNOSIS OF REFRIGERANT LINE OBSTRUCTION

A heating, ventilation, and air conditioning (HVAC) system of a building includes a refrigerant loop. A monitoring system for the HVAC system includes a monitoring device installed at the building. The monitoring device is configured to measure a first temperature of refrigerant in a refrigerant line located between a filter - drier of the refrigerant loop and an expansion valve of the refrigerant loop. The monitoring system includes a monitoring server, located remotely from the building. The monitoring server is con figured to receive the first temperature and, in response to the first temperature being less than a threshold, generate a refrigerant line restriction advisory. The monitoring server is configured to, in response to the refrigerant line restriction advisory, selectively generate an alert for transmission to at least one of a customer and an HVAC contractor

Assessment of Factors Contributing to Refrigerator Cycling Losses

Thermal mass effects, refrigerant dynamics, and interchanger transients are three factors affecting the transient and cycling performance of all refrigeration and air conditioning equipment. The effects of refrigerant dynamics, including refrigerant/oil solubility, off-cycle migration, and charge redistribution, were found to be the most important. These effects are quantified for a refrigerator instrumented with immersion thermocouples, pressure transducers, and microphones. The analytical methods, however, are applicable to other types of refrigeration and air conditioning systems, including those with capillary tube/suction line heat exchangers.Air Conditioning and Refrigeration Center Project 3

Development, Validation, and Application of a Refrigerator Simulation Model

This report describes the further development and validation of the Refrigerator/Freezer Simulation (RFSIM) model. The reports also describes the first major application of the model as an analysis tool for new refrigerator designs; several aspects of multi-speed compressor operation were examined with the model. Several improvements were made to the model that facilitated the validation process and the examination of multi-speed compressors: the model was made more general so that it could operate in numerous configurations in addition to the original design and simulation modes; many improvements were made in the modeling logic and robustness of the capillary tube-suction line heat exchanger model; and the equation-of-statebased property routines that calculated the thermodynamic properties were replaced with interpolation routines that were much faster. The RFSIM model, in design and simulation mode, was validated with data from two refrigerators. In both modes, the average model errors were less than ??5% for several important variables such as evaporator capacity and coefficient of performance. The errors of the simulation mode were reduced from the previous model validation primarily by using a different void fraction correlation in the refrigerant charge equations. The results from the validated RFSIM model indicate that a two-speed compressor could yield energy savings of 4% to 14% due to the increased steady-state efficiency at the low speed and an additional 0.5 to 4% savings due to the decreased cycling frequency. The results also showed that the capillary tube-suction line heat exchanger, when designed for the low speed, did not adversely affect the pull-down capacity when the compressor operated at the high speed. Lastly, it was found that a refrigerator operating at low ambient temperatures could actually benefit from a decrease in the condenser fan speed. This change in fan speed increased the evaporator capacity by reallocating charge to the evaporator and subsequently reducing the superheat at the evaporator exit.Air Conditioning and Refrigeration Project 6

Moisture transport processes and control of relative humidity in refrigerated facilities : a thesis presented in partial fulfilment of the requirements for the degree of Master of Engineering at Massey University, Palmerston North, New Zealand

Increasingly air relative humidity (RH) is becoming an important design and operational variable for refrigerated facilities. An integrated dynamic model of the main heat and moisture transfer mechanism in a refrigerated facility was developed. Specific features of the model that enabled RH to be predicted were: • Multiple air zones to represent variation of temperature and RH with position. • A single zone evaporator model with dehumidification based on a straight line approach to the saturation condition at the surface temperature. • Condensation and evaporation of water from surfaces and structures in the facilities. • Evaporator defrost assuming that a fraction of the defrost heat melts frost and the rest heats the evaporator and refrigerant mass. • Hot gas bypass with liquid refrigerant desuperheating to prevent the compressor operating into vacuum. • Moisture sorption by packaging associated with the product. The model was validated against data collected from a walk-in cool store 3.3m wide by 4.4m long by 3.0m high. The cool-store was cooled by an air cooled direct expansion HFC-134a refrigeration system with electric defrost, a suction line heat exchanger and electronic evaporation pressure regulating (EPR) valve for temperature control. To mimic the different design and operating conditions extra sensible and latent heat loads were provided by the cool store lights, up to 5 kW of electric heaters, and an ultrasonic humidifier. For the validation room trials fan speed, coil size, sensible load, latent loads and temperature set point were varied. Other conditions were held constant as far as possible and the room was operated for at least two defrost cycles. For the coolstore the model computed about 70 ordinary differential equations and more than 160 algebraic equations which were solved using Matlab 6.5, with the ODE45 solver. The measured and predicted store air temperature, RH, refrigerant suction and discharge temperatures and pressures showed good agreement for most of the trials during both pull-down and the mainly steady-state operation between defrosts. Differences in measured and predicted RH and refrigeration system operating conditions were largely explained by uncertainty in model input data, measurements and calibration; and imprecision of the actual refrigeration control system and particularly the hot gas bypass capacity control and the expansion valves. This suggests that the model is a useful tool for the design and optimisation of passive or active RH control strategies for refrigerated stores. Trials were also undertaken to quantify the effect of defrost frequency on the coolstore performance. Defrost efficiency and defrost duration were both proportional to defrost interval and doubled as defrost interval increased from 6 hours to 30 hours. For short defrost intervals; temperature control was poorer due to the frequent pull-downs. For longer defrost interval the room RH was lower and temperature control was poorer due to frost induced decline of evaporator performance The optimal defrost interval for the particular cool store was 8 to 12 hours. Overall energy use did not change significantly due to the use of EPR temperature control and the low latent heat loads used

Multiple Heat Exchangers Simulation Within the Newton-Raphson Framework

A general framework is proposed for simulating complex heat exchanger geometries in a manner suitable for sequential solution of the refrigerant- and air-side equations for mass, momentum and energy. The sequential solution enables the algorithm to be applied to a single module of a complex heat exchanger, and then integrated with other modules within a simultaneous equation solver employing a Newton-Raphson approach. This report also describes the integration of component subroutines into system simulation models for air conditioners and refrigerators. The modular approach is illustrated by describing its application to a dual-evaporator refrigerator simulation.Air Conditioning and Refrigeration Project 6

Refrigerant and Oil Migration and Retention in Air Conditioning and Refrigeration Systems

Air Conditioning and Refrigeration Project 16

Simulation of multi-deck medium temperature display cabinets with the integration of CFD and cooling coil models

This is the post-print version of the final paper published in Applied Energy. The published article is available from the link below. Changes resulting from the publishing process, such as peer review, editing, corrections, structural formatting, and other quality control mechanisms may not be reflected in this document. Changes may have been made to this work since it was submitted for publication. Copyright @ 2010 Elsevier B.V.In this paper, the model for the multi-deck medium temperature display cabinets is developed with the integration of CFD and cooling coil sub-models. The distributed method is used to develop the cooling coil model with the airside inputs from the outputs of the CFD model. Inversely, the airside outputs from the cooling coil model are used to update the boundary conditions of the CFD model. To validate this cabinet model, a multi-deck medium temperature display cabinet refrigerated with a secondary refrigerant cooling coil was selected as a prototype and mounted in an air conditioned chamber. Extensive tests were conducted at constant space air temperature and varied relative humilities. The cabinet model has been validated by comparing with the test results for the parameters of air at different locations of the flow path, and temperatures of refrigerant and food product, etc. The validated model is therefore used to explore and analyse the cabinet performance and control strategies at various operating and design conditions.DEFR

Simulating the Performance of a Heat Exchanger During Frosting

Factors affecting frost distribution are explored using a finite element model, developed and validated using a full-scale 8-row heat exchanger in a wind tunnel. The heat exchanger is typical of the type used in supermarket display cases; so face velocities and air inlet temperatures were varied from 0.5-2.3 m/s and 0 to -20 ??C, respectively, and inlet humidities from 70-90%. In order to focus on frost distribution, the prototype was designed to have a simple geometry and single-phase refrigerant to provide maximum certainty on parameters not directly related to frost. Measured and predicted total and sensible heat transfer agreed within RMS 6% and 8%, respectively, over the range of operating conditions. For latent heat, there was more scatter due to frost nonuniformities induced by the experimental apparatus. The simulation model was used to illustrate how the point of maximum frost thickness moved from the front to the rear of the heat exchanger, depending on face velocity, inlet humidity and fin surface temperature. Heat transfer and pressure drop were calculated from standard correlations, with fin thickness and tube diameter increasing as a function of frost thickness. The model was further extended to simulate the performance of the heat exchanger under the effect of a fan curve. A comparison is made between DX and indirect refrigeration system performance with respect to capacity, pressure drop and air flow variations under frosting conditions.Air Conditioning and Refrigeration Project 10