Absorption Refrigeration Unit Research Articles

A computational method for the methyl amine/H2 gas mixture concentrations in methyl amine/H2O/H2 absorption refrigeration units

Abstract Substitution of ammonia by methyl amine is proposed in neutral gas absorption refrigeration units using hydrogen as the pressure equalising gas. The throttling process of the conventional refrigeration units is being replaced by the diffusion of evaporated methyl amine through the hydrogen atmosphere. Accurate knowledge of the thermophysical properties of the working fluids, as well as heat and mass transfer characteristics, are required in the design of these units. This work reports an algorithm for the ‘psychrometric problem’, i.e. for the determination of the mass fraction of the methyl amine/hydrogen gas mixture from known ‘dry’ and ‘wet’ bulb temperatures. The values of the mixture's mass fraction are computed for various combinations of ‘dry’ and ‘wet’ bulb temperatures, due to analytical correlations concerning the thermodynamic and transport properties presented by this paper. The final results are given in psychrometric charts for pressures from 17.5 bar to 27.5 bar and temperatures between −30 and 50°C.

Thermodynamic analysis of combined diesel engine and absorption refrigeration unit—supercharged engine

This paper describes an attempt to calculate the amount of network, efficiency and also cooling capacity available in the exhaust gases of a supercharged diesel engine (ideal cycle) that is interfaced with an absorption refrigeration unit. An air standard cycle is assumed for the diesel engine. The variations of net work and efficiency of the diesel engine as a function of cycle pressure ratio and temperature ratio are calculated.

Thermodynamic cycle, correlations and nomograph for NH 3NaSCN absorption refrigeration systems

The detailed thermodynamic cycle of the NH 3NaSCN absorption refrigeration unit is presented, based on the thermodynamic properties of the working media. Correlations are developed, which express the coefficient of performance and the cooling capacity in terms of the required evaporation temperature, T ev, and the available ambient temperature, T amb. A nomograph is also presented, which shows in a compact form the behaviour of the NH 3NaSCN system and allows direct estimation of its main characteristics. It is concluded that if ( T amb − T ev) varies from 0 to 40°C, the theoretical coefficient of performance decreases linearly from 95 to 77%. For the same range of ( T amb − T ev) the theoretical cooling capacity varies from 1150 to 1300 kJ/kg NH 3 if T ev varies from 0 to −15°C. Under the conditions examined, for T amb − T ev > 23°C, the coefficient of performance of the NH 3NaSCN system becomes higher than that of the NH 3LiNO 3 system. The observed increase reached 4% at T amb − T ev = 40°C.

Simulation method of solar-driven absorption refrigerators for very low temperatures

A new solar-driven, NH 3H 2O absorption-refrigeration system is proposed, which may refrigeration temperatures as low as −60°C, i.e. much lower than the temperatures obtained by conventional solar-driven, absorption-refrigeration units. A method for simulating the operation of the proposed system has been developed and correlations have been derived which allow calculation of the characteristic quantities of the system in terms of ambient temperature and available solar radiation only. The correlations derived are valid for ammonia mass fractions 15% (weak solution), 40% (rich solution) and 98% (ammonia vapour), and for ambient temperatures from 5 to 35°C. The hour-by-hour performance of the proposed system has been predicted for operation in the Athens area throughout a typical year, using mean climatological data obtained by statistical processing of hourly measurements corresponding to about 20 years. The prediction showed that during the year: 1. (a) The lowest refrigeration temperature varies from −60 (in January) to −44°C (in July). 2. (b) For systems equipped with a triple heat exchanger the theoretical refrigeration power produced at about 1 p.m. varies from 45 W (in January) to 90 W (in July) per 1 m 2 of solar collector aperture while the theoretical coefficient of performance varies from 28.7% (in July) to 30.4% (in January).

The psychrometric problem for the evaporation of NH3 in NH3/H2 atmosphere in neutral gas absorption refrigeration units for pressures 17.5 to 27.5 bar

The present work considers the thermodynamic and transport properties of the real NH3/H2 gas mixture as a function of the state variables temperature, mass fraction and pressure. The relevant properties are presented in the form of analytical expressions, valid in the pressure range of 17.5 to 27.5 bar. The psychrometric problem is used to determine the mass fraction of the NH3/H2 gas mixture with the “dry” and “wet bulb” temperatures as input variables.

Use of helium instead of hydrogen in inert gas absorption refrigeration

The behaviour of helium in place of hydrogen as a pressure equalizing gas in triple-fluid absorption refrigeration units is examined. The main thermodynamic and transport properties of the real NH 3/He binary gas mixture are presented in the form of diagrams and analytical expressions, with the temperature and the mass fraction of the mixture as input variables. Among others, the mass or mole fraction of the NH 3/He gas mixture as a function of the dry and wet bulb temperatures has been plotted. A computer simulation has been developed to calculate the evaporation of NH 3 in a NH 3/He atmosphere within an insulated tube, allowing measurement of the composition of the mixture via temperature measurement. The results are compared with the corresponding values of evaporation of NH 3 in a NH 3/H 2 atmosphere.

Measuring the evaporation of NH 3 in triple-fluid gas absorption units

A computer simulation is presented which, among other functions, enables the gas concentrations in NH 3/H 2O/H 2 absorption refrigeration units to be determined by suitable temperature measurements. The gas concentration is determined by insulating the evaporator and measuring the inlet and outlet gas temperature, and the wall temperature at the outlet. Because of the pressure influence on the evaporation process, the operating pressure of the unit must also be measured. Numerical results are presented for pressures of 17.5, 20 and 25 bar.

A basic-2 program for the “psychrometric problem” of ammonia, in neutral gas absorption refrigeration units

The ‘psychrometric problem’ for small neutral gas absorption refrigeration units concerns among others the determination of the mass fraction of the NH 3/H 2 gas mixture, when the ‘dry’ and ‘wet bulb’ temperatures are known. By the present work, the values of the mass fraction for various combinations of ‘dry’ and ‘wet bulb’ temperatures are computed and the relevant program codes are given. The pressure range compiles the steps of 17.5 bar, 20 bar, 22.5 bar, 25 bar, and 27.5 bar, i.e. the usual operating conditions of these units.

A basic-2 program to compute the enthalpy/mass fraction diagram of the real NH 3/H 2 gas mixture at 25 bar, for neutral gas absorption refrigeration units (technical note)

The thermodynamic properties of the NH 3/H 2 gas mixture, namely the enthalpy, the specific volume of the mixture and the NH 3 ‘partial pressure’ can be presented in an enthalpy/mass fraction diagram as a function of the state conditions (temperature, mass fraction, total pressure) to facilitate the calculation of the evaporation of NH 3 taking place in small absorption refrigeration units. A method and a computer program have been developed giving the desired quantities, starting from the corresponding data of the real components H 2 and NH 3 and using the method of the second virial coefficients. The relevant results are valid for a total pressure of 25 bar and temperatures between −40°C to +50°C.

Performance of an aqua-ammonia absorption solar refrigerator at sub-freezing evaporator conditions

Ice making by means of a solar-assisted aqua-ammonia absorption refrigeration unit at subfreezing evaporator conditions is considered in terms of its technical and economic feasibility. A computer-aided thermodynamic analysis is performed for various ranges of operation parameters, three climatic locations varying from 15°N to 43°N latitude and four solar collector types, i.e. flat plate, compound parabolic collector, and east-west and north-south axis tracking concentrators. In order to use an air-cooled condenser, the simultation is predicted on an absorber temperature of 35°C and a condenser temperature of 38°C. The results indicate that for generator pressures of 1.02 to 2.07 MPa, generator temperatures greater than 120°C are required. At these conditions, the COP is on the order of 0.5 and a conventional flat plate collector is not satisfactory. For the three climates, the compound parabolic collector (CPC) has a higher output than an east-west axis tracking concentrator but less than the north-south tracker. The shape of the insolation curves at the lower latitudes causes difficulties in obtaining optimal collector sizes. The cost comparison between the CPC and north-south tracker indicates that overall system costs will range between $85 and $155,000 (1981) for a one ton of ice per day system. The projected costs per ton for a 25-year life are in the range of $10 to $20, which are favorable compared to the cost of domestic ice. † Graduate Research Assistant. ‡ Professor.

Performance of solar NH 3/H 2O absorption cycles in the athens area

The performance of solar driven NH 3/H 2O absorption units, operating in conjunction with high and intermediate temperature solar collectors in Athens, is predicted along the typical year, in the cases (a) of absorption refrigeration units working as refrigerators, (b) of absorption refrigeration units working as heat pumps and (c) of reversed absorption units working as heat transformers. In all cases, the operation of the units and the related thermodynamics are simulated by suitable computer codes, and the required local climatological data (i.e. the incident solar radiation and the ambient temperature) are determined by statistical processings of related hourly measurements over a considerable number of years. It is found that in the case of the refrigerator, for operation over the whole year, the theoretical coefficient of performance varies in the range from 72 to 75% and a maximum theoretical specific cooling power of 223 W/m 2 is observed on July at 13 hrs. In the case of the heat pump, for operation from November to April, a maximum theoretical heat gain factor of about 170% is obtained on December with corresponding specific heat gain power amounting to 213 W/m 2 at 14 hrs, while a maximum theoretical specific heat gain power of 344 W/m 2 is observed on April at 13 hrs with a corresponding heat gain factor of about 165.5%. Lastly, in the case of the heat transformer, for operation over the whole year, a maximum theoretical heat gain factor of about 48.3% is observed during winter at about 13 hrs but with very small specific heat gain power, while a maximum specific heat gain power of 175 W/m 2 is obtainable on July at noon with a corresponding heat gain factor of 44.5%.

Development of absorption refrigeration units for cold storage of agricultural products

To meet the requirements of continuous refrigeration installations, powered by low-grade heat sources, e.g. waste heat, flat-plate solar collectors or solar ponds, and providing cooling or refrigeration at subzero (°C) temperatures, rational selection of suitable refrigerant-absorbent pairs is necessary. It is demonstrated that the use of R22 and suitable organic solvent systems is desirable in absorption refrigeration installations. Since no data were available on the vapour-liquid equilibria of these combinations, they were measured over a broad range of pressures, temperatures and concentrations. A rational selection of the optimal combinations was made using results obtained from a computerized simulation model giving the performance characteristics of a specific refrigeration system operating with selected refrigerant-absorbent combinations. An efficient thermodynamic cycle has been constructed for the chosen refrigerant-absorbent pairs. Within the region of feasible operation of the cycle, optimal operating conditions are determined. In selecting these conditions it is also necessary to take into account the heat transferred in the heat exchangers of the unit and the operating stability. On the basis of the research performed a 15 ton refrigeration prototype unit for the storage of agricultural products has been constructed and is in the final stages of being run in. It operates at a coil temperature of −3°C and maintains the storage chamber at 5°C. Hot water is supplied at 95–98°C, and cooling water is provided at 26°C.

Evaporation of NH3 in NH3/H2 atmosphere by 25 bar for neutral gas absorption refrigeration units

The present work considers the evaporation process ofNH 3 inNH 3/H2 atmosphere, that takes place in neutral gas absorption refrigeration units. The data obtained are used to calculate the “psychrometric problem” and the evaporation process within an insulated tube. The “psychrometric problem” deals with the determination of the mass fraction of theNH 3/H 2 gas mixture from known “dry” and “wet bulb” temperatures for Lewis numbers not equal to one. The results are in good agreement with some existing experimental data.

Thermodynamic Analysis of a Solar Zeolite Refrigeration System

A solar zeolite-water absorption refrigeration unit was studied. Thermodynamic expressions were derived to predict the system performance. The operating range and optimum design parameters for the zeolite system were determined. The main parameters governing performance were: solar collector type, ambient temperature, and absorber properties. Results are shown for various solar collector types and various zeolite types. A desorption initiation temperature is shown to exist. The analysis is not based on empirical heat of absorption data, but rather on a zeolite properties relationship (log Pv versus 1/Tz) to determine system performance.

Utilization of secondary energy resources to produce higher vacuums

A major reserve for savings in manufacturing is the use of secondary energy resources (SER) to produce refrigeration in absorption refrigeration units (ARU). One possible field of application of refrigeration is the development of the required vacuum in vacuum towers. This paper proposes a scheme for the production of refrigeration in the form of chilled water, using an ARU, by utilization of the SER of the head stream from the atmospheric tower of an ELOU-AVT-6 unit, with an intermediate heat carrier loop added to be able to use standardized ARUs.

Analysis of a Combined Gas Turbine and Absorption-Refrigeration Cycle

An appreciable improvement in the performance of gas turbines operating with ambient temperatures above 90 deg F could be achieved by combining an absorption-refrigeration unit to the power cycle. The thermal energy in the exhaust gases from the turbine is used to operate the refrigeration unit, which in turn cools the air prior to entering the compressor. This reduction in compressor-inlet temperature results an average improvement of about 7–9 points in the thermal efficiency of the combined cycle as well as an increase in the specific output. The analysis includes the effect of different cycle parameters such as compressor pressure ratio, maximum cycle temperature, and regeneration.

Design efficiency indices of the operation of a single stage absorption refrigeration unit using an ammonia-water mixture

Recent growth in r e f r i ge ra t i on r equ i r emen t s has been t remendous . E lec t r ic power consumption to mee t all these r equ i r emen t s using only c o m p r e s s i o n r e f r ige ra t ion machines would be ve ry great . At the s ame t ime many plants have p rac t i ca l ly unlimited sources of waste heat. P r e l i m i n a r y calculat ions show that uti l ization of this was te heat would more than sa t i s fy the i r r e f r ige ra t ion requ i rement s . It is impor tan t fo r r e s e a r c h e r s and des igners to know what levels of waste heat can be used to obtain r e f r i ge r a t i on at the requ i red t e m p e r a t u r e s and what eff ic iency can be achieved by an absorpt ion r e f r i g e r a t ion unit. This a r t i c l e is devoted to this subject . F igure 1 is a schemat ic , while Fig. 2 is the graphical r ep resen ta t ion on the i -~ d iagram, of a single s tage absorpt ion r e f r i ge ra t i on unit operat ing on an a m m o n i a water mixture . Concentra ted ammonia solution at a concentra t ion of ~r en te r s gene ra to r I which is supplied with heat qn" This heat input genera tes ammonia and wate r vapor with a concentrat ion of ~ and a dilute solution (concentration ~a)" The genera to r vapor is pa r t i a l ly condensed in II to inc rease vapor concentrat ion f rom ~5 to ~7 = ~d" The reflux condenser is cooled by wate r removing heat qR.

Experiments with solar-energy utilization at Dacca

This paper presents results of studies at Dacca to utilize solar energy for water heating and refrigeration. Measured values of radiation intensity with an Eppley pyrheliometer were generally in agreement with standard theoretical values, except in months when either the diffuse radiation and scattered cloud reflection or the humidity of the atmosphere was higher than average. The corrugated-sheet, flat-plate collector was found to be an inexpensive and satisfactory solar water heater. It was possible to predict the temperature rise in the system with thermosyphon circulation. An intermittent ammonia-water absorption refrigeration unit was constructed and tested with a 4 1 2 - foot parabolic reflector and an artificial electric heater. Performance tests indicated that it has practical possibilities of application in rural areas but that further tests are needed for an improved design of evaporator and absorber units.

CORROSION COMMENTARY

WATER SERVICE PROBLEM. An unusual solution was found to a perplexing problem in the water treatment of an air‐conditioning system in Lancaster, Pennsylvania, U.S.A. The system, comprising a 150‐ton compressor‐type comfort cooling system and a 510‐ton Carrier absorption refrigeration unit, installed in offices and plant in June 1955 and June 1956, respectively, is serviced by separate cooling towers. Within three weeks of the 510‐ton unit going into operation, serious operating difficulties, which virtually shut down the air‐conditioning equipment, developed. Water Service Laboratories, who are specialists in water problems, were called in to survey the cooling water problem. Samples of make‐up water from each of the two cooling towers located on the roof of the plant were sent to the laboratories for analysis.