A Century of Chiller Technology

A convergence of technologies had to occur to make the modern, high-efficiency centrifugal chiller a reality. To appreciate the technology fully, we must go back in history and understand the origins of the air conditioning and refrigeration industry. Along the way, we will find an important diversion in aerospace and the critically important centrifugal compressor. Ultimately, we will find that the modern chiller is a testament to advanced technology that was developed in multiple fields.

Some of the first advances in and applications of modern industrial refrigeration were in the United States. In May 1922, Willis Carrier revealed the “Centrifugal Refrigeration Machine” – a very early incarnation of what we now call a chiller [1]. The first installation went to a Philadelphia candy manufacturer; it’s interesting to know that the birth of modern refrigeration and air conditioning started on a large scale. Back in those days, economy of scale enabled the technology to be developed. It was not until a decade later that the core technology began to be adopted into compact units that could be used in smaller businesses such as boutique shops. It took several more decades for smaller residential air conditioners to take off commercially.

Shown in the photograph below is Carrier’s first centrifugal chiller in his New Jersey factory [1].

First Centrifugal Chiller
Photo from [1]
The size of this machine is evident, as is the fact that its design, at the time, necessitated components be spread out in space for assembly and maintenance. By modern standards, the same footprint space could be used to accommodate a modern chiller with over 500 refrigeration tons in capacity. By comparison the original design has less than 100 refrigeration tons of capacity.

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Modeling a Ground Source Heat Pump

Ground source heat pumps (GSHP) are one of the fastest growing applications of renewable energy in the world, with annual increase of 10% in about 30 countries over the past 15 years.  Its main advantage is that it uses normal ground or ground water temperatures to provide heating, cooling and domestic hot water for residential and commercial buildings. GSHP’s are proving to be one of the most reliable and cost-effective heating/cooling systems that are currently available on the market and have the potential of becoming the heating system of choice to many future consumers, because of its capacity for providing a variety of services such as heat generation, hot water, humidity control, and air cooling. Additionally,  they have the potential to reduce primary energy consumption, and subsequently provide lower carbon emissions, as well as operate more quietly and have a longer life span than traditional HVAC systems. The costs associated with GSHP systems are gradually decreasing every year due to successive technological improvements, which makes them more appealing to new consumers.

The basic purpose of a GSHP is to transfer heat from the ground (or a body of water) to the inside of a building. The heat pump’s process can be reversed, in which case it will extract heat from the building and release it into the ground. Thus, the ground is the main heat source and sink. During winter, the ground will provide the heat whereas in the summer it will absorb the heat.

A GSHP comes in two basic configurations: ground-coupled (closed-loop) and groundwater (open loop) systems, which are installed horizontally and vertically, or in wells and lakes. The type chosen depends upon various factors such as the soil and rock type at the installation, the heating and cooling load required, the land available as well as the availability of a water well, or the feasibility of creating one. Figure 1 shows the diagrams of these systems.

Two Basic Configurations
Figure 1. Two Basic Configurations of GSHP Systems. SOURCE: [1]
In the ground-coupled system (Figure 1a), a closed loop of pipe, placed either horizontally (1 to 2 m deep) or vertically (50 to 100 m deep), is placed in the ground and a water-antifreeze solution is circulated through the plastic pipes to either collect heat from the ground in the winter or reject heat to the ground in the summer. The open loop system (Figure 1b), runs groundwater or lake water directly in the heat exchanger and then discharges it into another well, stream, lake, or on the ground depending upon local laws. Between the two, ground-coupled (closed loop) GSHP’s are more popular because they are very adaptable.
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Vapor-Compression Refrigeration Systems

Present day refrigeration is viewed as a necessity to keep our popsicles cold and our perishables fresh. But have you ever wondered what people did to keep their food from spoiling hundreds or even thousands of years ago? Or just what goes into a refrigerator system today? In this blog, we’ll take a look at how refrigeration works; the history behind it; and examine the cycle, working fluids, and components.

Schematic of Refridgeration System in AxSTREAM NET
Figure 1. Modern-day Refrigeration System in AxSTREAM NET
Introduction

Refrigeration is based on the two basic principles of evaporation and condensation. When liquid evaporates it absorbs heat and when liquid condenses, it releases heat. Once you have these principles in mind, understanding how a refrigerator works becomes much more digestible (pun intended). A modern-day refrigerator consists of components such as a condenser, compressor, evaporator and expansion valve, as well as a working fluid (refrigerant). The refrigerant is a liquid which as enters the expansion valve the rapid drop in pressure makes it expand, cool, and turn into a gas. As the refrigerant flows in the evaporator, it absorbs and removes heat from the surrounding. The compressor then compresses (as the name suggests) the fluid, raising its temperature and pressure. From here, the refrigerant flows through the condenser, releasing the heat into the air and cooling the gas back down to a liquid. Finally, the refrigerant enters the expansion valve and the cycle repeats. But what did we do before this technology was available to us?

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Unsteady Flow Simulation in Hydraulic Systems

[:en]An unsteady flow is one where the parameters change with respect to time. In general, any liquid flow is unsteady. But if a hydraulic system is working at constant boundary conditions, then the parameters of the fluid flow change slowly; thus this flow is considered steady. At the same time, if the parameters of the fluid flow oscillate over time relative to some constant value, then it called quasi-steady flow 1.

In practice, most fluid flows are steady or quasi-steady. Examples of the three flows are presented in Figure 1. Steady flow is presented by a simple pipe. The quasi-steady flow is represented by a sharpened edge channel. The unsteady flow is presented by an outflow from a reservoir.

Figure 1 - Different Types of Fluid Flow
Figure 1 – Different Types of Fluid Flow
Different Cases of Unsteady Flow

During operations, hydraulic systems act for long intervals at steady conditions which are called operating modes. Change between two different operating modes occurs over a short time interval (called a transient mode). If any hydraulic system works more than 95% of the time at these operating modes though, why is the unsteady flow is so important? Because the loads depend on time intervals. If the load is less, then the maximum system pressure is higher. Read More

Preventing Choke and Surge in a Compressor

Turbo Compressors are used to increase the pressure of a gas, which are required in propulsion systems like a gas turbine, as well as many production processes in the energy sectors, and various other important industries such as the oil and gas, chemical industries, and many more.

Such compressors are highly specific to the working fluid used (gas) and the specific operating conditions of the processes for which they are designed. This makes them very expensive. Thus, such turbo compressors should be designed and operate with high level of care and accuracy to avoid any failure and to extract the best performance possible from the machine.

Axial Compressor and Centrifugal Compressor in AxSTREAM
Figure 1 (A) Axial Compressor (B) Centrifugal Compressor in AxSTREAM®

Turbo Compressor Characteristic Curves

The characteristic curves of any turbo compressor define the operating zone for the compressor at different speed lines and is limited by the two phenomenon called choke and surge. These two opposing constraints can be seen in Figure 2.

Choke conditions occurs when a compressor operates at the maximum mass flow rate. Maximum flow happens as the Mach number reaches to unity at some part of the compressor, i.e. as it reaches sonic velocity, the flow is said to be choked. In other words, the maximum volume flow rate in compressor passage is limited by limited size of the throat region.  Generally, this calculation is important for applications where high molecular weight fluids are involved in the compression process.

Surge is the characteristic behavior of a turbo compressor at low flow rate conditions where a complete breakdown of steady flow occurs. Due to a surge, the outlet pressure of the compressor is reduced drastically, and results in flow reversal from discharge to suction. It is an undesirable phenomenon that can create high vibrations, damage the rotor bearings, rotor seals, compressor driver and affect the entire cycle operation.

Compressor Performance Curve
Figure 2 Compressor Performance Curve

Preventing Choke and Surge Conditions

Both choke conditions and surge conditions are undesirable for optimal operation of a turbo compressor.  Each condition must be considered during design to ensure these conditions are prevented. Read More

Industrial Refrigerator Modeling

Refrigerators are an integral part of everyday life to the point where it is almost impossible to image our day without them. As in our everyday life, refrigeration units are also widely used for industrial purposes, not only as stationary units but also for transporting cold goods over long distances. In this blog, we will focus on the simulation and modeling of such an industrial refrigeration unit.

Picture 1 - Industrial Refrigerator
Industrial Refrigerator

Like any stationary refrigeration unit, a unit used for cooled transportation includes an intermediate heat exchanger, a pump, an evaporator, a compressor, a condenser, and a throttle. The most common refrigeration scheme uses three heat fluids in the industrial refrigeration cycle. There is Water, which is used for heat removal from Refrigerant- R134A and Propylene glycol 55%. These other fluids are used as intermediate fluids between the refrigerator chamber and refrigerant loop. The working principle of all fridge systems are based on the phase transition process that occurs during the refrigerator cycle shown in Figure 1. The propylene glycol is pumped into the evaporator from the heat exchanger, in which it cools and transfers heat to the refrigerant. In the evaporator, the refrigerant boils and gasifies during the heat transfer process and takes heat from the refrigerator. The gaseous refrigerant enters the condenser due to the compressor working, where its phase transition occurs to the liquid state and cycle repeats.
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An Overview of Axial Fans

[:en]Axial fans have become indispensable in everyday applications starting from ceiling fans to industrial applications and aerospace fans.  The fan has become a part of every application where ventilation and cooling is required, like in a condenser, radiator, electronics etc., and they are available in the wide range of sizes from few millimeters to several meters. Fans generate pressure to move air/gases against the resistance caused by ducts, dampers, or other components in a fan system. Axial-flow fans are better suited for low-resistance, high-flow applications and can have widely varied operating characteristics depending on blade width and shape, number of blades, and tip speed.

Fan Types

The major types of axial flow fans are: propeller, tube axial, and vane axial.

  • – Propellers usually run at low speeds and handle large volumes of gas at low pressure. Often used as exhaust fans these have an efficiency of around 50% or less.
  • – Tube-axial fans turn faster than propeller fans, enabling operation under high-pressures 2500 – 4000 Pa with an efficiency of up to 65%.
  • – Vane-axial fans have guide vanes that improve the efficiency and operate at pressures up to 5000 Pa. Efficiency is up to 85%.
Types of Fans
Figure 1 Different Types of Axial Fans
Aerodynamic Design of an Axial Fan

The aerodynamic design of an axial fan depends on its applications. For example, axial fans for industrial cooling applications operate at low speeds and require simple profile shapes. When it comes to aircraft applications however, the fan must operate at very high speeds, and the aerodynamic design requirements become significantly different from more traditional fan designs. Read More

Modern Approaches and Significance of Multiphase Flow Modeling

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Introduction

Corresponding with the development of industrial technology in the middle of the nineteenth century, people  dealt with multiphase flows but the decision to describe them in a rigorous mathematical form was first made only 70 years ago. As the years progressed, development of computers and computation technologies led to the revolution in mathematical modeling of mixing and multiphase flows. There are a few periods, which could describe the development of this computation:

«Empirical Period» (1950-1975)

There were a lot of experiments which were done during this period. All models were obtained from experimental or industrial facilities which is why using them was difficult for different cases.

«Awakening Period» (1975-1985)

Because of sophisticated, expensive and not universal experiments, the researchers’ attention was directed to the physical processes in multiphase flows.

«Modeling Period» (1985-Present)

Today, the models for multi-flow calculation using the equations of continuity together with equations of energy conservation are obtained, which allow describing phase’s interaction for different flow regimes. (A.V. Babenko, L. B. Korelshtein – Hydraulic calculation two phase gas liquid course: modern approach // Calculations and modeling journal. – 2016. – TPА 2 (83) 2016. – P.38-42.)

Technology Development

Since the time of industrial development, installation designs have undergone great changes. For example, there are shell and tube evaporators for freeze systems where the heat transfer coefficient has increased 10 times over during the last 50 years. These results are a consequence of different innovation decisions. Developments led to research into mini-channels systems, which is the one of the methods to increase intensification of phase transition. Research has shown that heat exchange systems with micro and nano dimensions have a much greater effect than the macrosystems with channels dimensions ≤3-200 mm.

In order to organize fundamental research, it is very important to understand hydro, gas dynamics and heat changes in two-phase systems with the phase transition. At present, the number of researchers using advanced CFD-programs has increased. Our team is one of the lead developers of these program complexes.

Mathematical modeling of compressible multiphase fluid flows is interesting with a lot of scientific directions, and has big potential for practical use in many different engineering fields. Today it is no secret that environmental issues are some of the most commonly discussed questions in the world. People are trying to reduce the emissions of combustion products. One of the methods to decrease emissions is the organization of an environmentally acceptable process of fuel burning with reduced yields of nitrogen and sulfur. The last blog (https://blog.softinway.com/en/modern-approach-to-liquid-rocket-engine-development-for-microsatellite-launchers/) discussed numerical methods, which can calculate these tasks with minimal time and cost in CFD applications.

Waste Heat Boiler
Picture 1 – Waste heat boiler http://tesiaes.ru/?p=6291

For more effective use of energy resources and low-potential heat utilization, the choice of the Organic Rankine Cycle (ORC) is justified. Due to the fact that heat is used and converted to mechanical work, it is important to use a fluid with a boiling temperature lower than the boiling temperature of water at atmospheric pressure (with working flow-boiling temperature about 100⁰C). The usage of freons and hydrocarbons in these systems makes a solution impossible without taking into account the changes of working fluid phases. Read More

An Introduction to Accurate HVAC System Modeling

[:en]HVAC (Heat, Ventilation and Air Conditioning) is all about comfort, and comfort is a subjective feeling associated with many parameters like air quality, air temperature, surrounding surface temperature, air flow and relative humidity. For example, while it is easy to understand how the temperature of the air in your living impacts how good you feel, the surfaces with which you are in contact also strongly affect your comfort. For example, last night I got out of bed to clean up after my dog who thought it would be a good idea to swallow (and give back) her chew toy. If I was wearing my slippers, it would have been much easier to go back to sleep between the warm bed sheets without the discomfort of waiting my cold feet warm up to normal temperature.

Speaking of sleep discomfort, many stem from HVAC imbalances.  If you wake up in the middle of the night quite thirsty, then you should probably check how dry your bedroom is. The recommended range is 40-60% relative humidity. A higher humidity puts you at risk for mold while lower humidity can lead to respiratory infections, asthma, etc.

Now that we know how HVAC contributes to our comfort, let’s look at the HVAC unit as a system and see its role, functioning and simulation at a high level. The following examples provided are for a house, but similar concepts apply to residential buildings, offices, and so on.

Controlling Temperature

The easiest parameter to control is the air temperature. It can be set by a thermostat and regulated according to a heating or cooling flow distributed from the HVAC unit to the different rooms through ducting. Without the introduction of thermally-different-than-ambient air, the house will heat or cool itself based on a combination of outside conditions and how well the building is insulated. Therefore, to keep a constant temperature a certain amount of energy must be used to provide heating (or cooling) at the same rate the house is losing (or gaining) heat.  This is a match of the house load and heating/cooling capacity. Figure 1 provides a graph of the energy needed.

Illustration of dependency of house load and heating capacity on outside temperature
Figure 1 Illustration of dependency of house load and heating capacity on outside temperature

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Identifying Compressor Problems

[:en]Centrifugal Compressor for Refrigeration Because the most vital part of a refrigeration and HVAC system is to function optimally, compressors are used to raise the temperature and pressure of the low superheated gas to move fluid into the condenser. Consequently, refrigeration compressors must be properly maintained through regular maintenance, testing and inspection. There are a couple conditions which would indicate compressor problem or failures. However, with the right supervision it is possible to avoid further damage. Through this post we will identify and discuss some of these conditions:
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