Chiller

A chiller is a device that " cold " - produced - a state below the ambient temperature. If an object with the cold spot is brought into contact, this object is removed from heat.

Generally

Many refrigerating machine based on a thermodynamic cycle, in which received while supplying external heat energy at a point below the ambient temperature and transferred to another place at a higher temperature. In this sense, a chiller of a heat pump is similar. In order to realize to use:

• So-called cold steam equipment in which the property of materials used to have different boiling and condensation temperatures, at different pressures. The fabrics used in this way are known as refrigerants. The working range is limited by the achievable boiling or condensation temperature of the refrigerant.
• The Joule -Thomson effect ( JTE ), which causes real gases cool during the expansion ( throttling). This effect is, for example, the basis of the Linde process. Able to multi-tier applications very low temperatures in the industrial sector, eg for air liquefaction, can be achieved.

There are also methods that do not involve gases and moving parts.

History

The cooling by pumping air from a half-filled with diethyl ether glass flask was already discovered in the mid -18th century, but was still no real gain. The first working refrigeration machine in the world in 1845 by the American physician John Gorrie built in Florida, who was looking for ways to improve the health outcomes for hospital patients in hot and humid Florida. After former medical school of thought was " bad air " a substantial factor disease, and brought in from the northern Great Lakes winter ice, which represented the only possibility for cooling, was in Florida because of the large transport losses very expensive. Gorries machine that used the reverse principle of the Stirling engine, used for ice and also for space cooling ( air conditioning). A prototype was built. However, the machine was a financial failure ( Patent Application 8080, May 6, 1851). Gorrie died a few years later, penniless and ridiculed. Only in the 1870s chillers were economically, the first major users were breweries that also could brew so no natural cool cave systems bottom-fermented, more durable lager after lager method. One of the first major manufacturers included the German industrialist Carl von Linde.

Implementation of the system for heat transfer

The " generated " by the refrigerator cold can be used for technical process purposes, for air conditioning for ice ( ice rinks ), or for the preservation or refrigeration of food. The heat absorption can be direct or indirect way. In the case of indirect cooling of a cooling medium ( chilled water or brine ) is used, which is cooled by the evaporating refrigerant in the first heat exchanger and absorbs heat from the medium to be cooled in the second heat exchanger. For direct use, a heat exchanger is used, which leads on the one hand, the evaporating refrigerant on the other side of the medium to be cooled.

Types

The main difference between compression and sorption is that is completely fed in the former the energy required than mechanical work, in the latter case, however, in the form of heat. The latter require mechanical work only to overcome the internal pressure losses.

The efficiency is usually related to compression chillers on the electrical drive energy, which results in significantly better figures compared to sorption. A comparison of this kind is, however, inadmissible as mechanical or electrical drive energy in nature is not available, but is generated from fossil or renewable sources with losses ( converted ) must be, which is reflected in the energy price. Incorporating a these losses, the efficiency of sorption are also of value comparable if not better.

The efficiency is called for chillers COP.

Absorption chillers

The absorption chiller has an additional solvent and a refrigerant circuit. The working medium consisting of two components, a solvent, and the refrigerant. The refrigerant must be completely soluble in the solvent. Technically common are absorption chillers with water as the refrigerant and lithium bromide aqueous bromide (LiBr ) solution as the solvent. By vacuum evaporation operating temperatures of the water are accessible to about 3 ° C. Lower temperatures can reach absorption chillers that use NH3 as refrigerant and water as solvent. In a large scale used ammonia absorption refrigeration plants evaporating temperatures of -70 ° C can be achieved.

In sorption comes as more heating power could be added the heat of sorption, which must be removed from the wastewater or adsorber.

Adsorption chillers

The adsorption chiller operates at a fixed solvent, the " adsorbent ", in which the refrigerant is ad or desorbed. The process is fed in the desorption and removed in the adsorption heat. Since the adsorbent can not be circulated in a loop, the process can only discontinuously. Therefore, two chambers with the adsorbent may be used in which in a working cycle ( 6-10 minutes), the adsorption and desorption parallel. After completion of the work cycle heat supply and heat dissipation for the two chambers to be replaced ( switching, about 1 min. ). Then, the adsorption and desorption starts again parallel. This allows an almost uniform cooling can be ensured.

Diffusion absorption refrigerator

The diffusion absorption refrigeration system operates similarly to the absorption chiller, the pressure change is, however, realized as partial pressure change. For this, a third component of the working fluid is necessary an inert gas. Their advantage lies in the fact that the pressure body is hermetically sealed and not releasable seals are required, and that the apparatus operates silently. The technique is used for example in camping and hotel refrigerators.

Compression refrigeration systems

The compression refrigerating machine is equipped with a mechanical compressor ( compressor ) and a throttle device ( expansion valve, for example ). A compression and an expansion element, as well as two heat exchangers, which are connected together in a loop so that the heat exchangers are connected at both ends between the compression and expansion element.

In the cycle, the refrigerant vapor is sucked in by the compressor (compressor ), and compacted (drive power). In the downstream heat exchanger ( condenser), the refrigerant condenses. The liquid refrigerant is routed to a flow regulator and relaxed. Upon expansion, the refrigerant pressure is decreased, the refrigerant is cooled and partially evaporated. In the second heat exchanger ( evaporator), the refrigerant absorbs heat by evaporation of the fed from the cooling chamber to ( cooling capacity / evaporation heat). The compressor sucks the refrigerant evaporated again and the cycle is closed.

For the operation of the chiller must be supplied in the form of mechanical work according to the second law of thermodynamics, energy from the outside. The heat of condensation at the condenser output is the sum of the absorbed heat energy to the evaporator, the drive power and the operational losses of the low-temperature insulation and friction losses.

This technique is widely used in household refrigerators, freezers and freezers, dispensing systems, cold storage, air conditioning, Kunsteispisten, abattoirs, breweries and chemical industries.

Schukey the motor has a simple structure and makes use of air as refrigerant.

Steam jet refrigeration system

The steam jet refrigeration system is a thermal cooling system that uses water vapor as propellants, refrigerants and refrigerant. By the expansion and relaxation of a steam jet, a vacuum is generated and drawn in the water vapor from an evaporator. By the evaporation of the water reservoir is cooled in the evaporator, and can thus be used as a cooling medium.

Joule- Thomson effect ( JTE ), Linde process

For cooling, the temperature of a gas ( eg, air, helium ), which will not condense in the work area, lowered by throttling. With the JTE can pressure difference ( air about 1/4 K / bar, CO2 about 3/4 K / bar ) can be achieved at the throttle a cooling of about 0.4 K per bar. Although apparently this effect is very small, this will also enable low temperatures down to near absolute zero reach. Plants are often run in several stages.

The apparatus representation of a Joule-Thomson system is similar to a compression chiller, the heat exchangers are not built as a condenser or evaporator. For energy optimization, it is necessary, before the expansion valve ( throttle ) pre-cool the gas in a recuperative ( counter-current ) heat exchanger with the returning of the cooler gas.

1895 Carl Linde has such a facility used for air liquefaction and quite large amounts of liquefied ( 1 bucket / h) in air. That based on the Joule- Thomson effect, technological methods for air liquefaction and separation has since been known Linde process.

The decisive factor is however for cooling by the Joule -Thomson process, the output temperature is less than the inversion temperature of the respective gas. This is air at about 659 ° C, hydrogen at 80 ° C and helium at -239 ° C. When a gas is relaxed below its inversion temperature, it cools down, it is expanded above its inversion temperature, it heats up. In order to cool a gas after the JT process, therefore, the outlet temperature must be below the inversion temperature ( a van der Waals gas: Ti = 6.75 · Tk = 2a / Rb Tk = critical temperature, a = Van der Waals constant, b = covolume ) lie.

Pulse tube cooler

A pulse tube refrigerator is a refrigerating machine, whose principle of operation is about one Stirling engine, but which requires no mechanical moving parts. This very compact cooling heads are possible and the attainable minimum temperature is not limited by the mechanical friction heat of these parts. Than previously lowest temperature of 1.3 C ( = -272 ° C) have been achieved.

Thermoelectric effect, Peltier element

For cooling (or heating), a Peltier element may be used which is electrically operated, and requires no refrigerant. For large temperature differences ( 50 .. 70 K) but the cooling capacity drops to zero. For higher temperature differences using pyramidal multistage structures.

This technique is used for temperature stabilization of semiconductor lasers and sensors, in car coolers, thermal cyclers (PCR) and for cooling of image sensors in cameras from infrared to ultraviolet.

Magnetic cooling

Another cooling method is based on the magnetic properties of certain substances. In magnetization some substances release heat, they are called then magnetocalorische substances. In the magnetic cooling the substance is placed in a magnetic field, it heats up; the heat is mostly dissipated here by means of a cooling liquid. The again, brought to ambient temperature material now leaves the magnetic field and demagnetized in the area to be cooled. In the demagnetization of the material absorbs heat. Energy must be supplied to remove the magnetic material from the magnetic field.

Such cooling systems are generally more efficient than systems that use steam, but more expensive, because magnetocalorische suitable materials, such as gadolinium, are expensive.

Evaporative cooling

The evaporative cooling, and adiabatic or adiabatic cooling, is one of the oldest and best cooling methods. It is a natural defect, which is caused by evaporation of water. The required energy is extracted from the environment in the form of heat. Everyone is familiar with practical effects and applications such as the butter cooler, earthen vessels, which are kept moist and allow on the open-pore surface evaporation. Or the ability to determine with a damp highly elongated finger the direction from which the wind comes. It is this side of the finger is cold. The effect of cooling by the surrounding air temperature and relative humidity condition dependent. With relative humidity close to 100 %, that in foggy weather, the effect is almost not detectable. However, the lower the relative humidity, the higher the potential of further moisture absorption, the more water can thus evaporate. All air changes of state can be represented in the Mollier hx diagram. The total energy content of the air is given in kg kJ /. Since the total energy content does not change during the evaporative cooling, the state change direction is therefore always on the Enthalpielinien from left top right bottom. With relative humidity of 100 % to reach the saturation line. Example: Start point 21 ° C / 40 % RH > enthalpy = 36.7 kJ / kg. Extending the Enthalpielinie intersects the dew point at 13 ° C / 100 % RH. A lower temperature than 13 ° C can be here by the sum of evaporation not reach.

Process technology, the effect is, for example, used in wet cooling tower. This may in Central Europe outside temperature provide a cooling water temperature of about 27 ° C., generally at 32 ° C, ie a temperature well below the ambient.

The evaporative cooling is reinforced by a phase transition heat transfer process from high to low temperature, and thus a self-running, " Clockwise " cycle. Therefore, except for the transport of air and water, no mechanical, electrical or thermal energy is required.

Coefficient of performance

The thermal efficiency of a cooling or heating capacity based on the amount mechanical work is called the coefficient of performance or COP. In older literature, the symbol ε is used for the coefficient of performance. In the current standardization, the English names are used; for a refrigeration system, the term EER ( energy efficiency ratio) and used for heat pump COP (Coefficient of Performance).

For the cooling system with use of the cooling capacity QK applies:

The heat capacity is the sum of the added cooling capacity and the technical work, such that:

.

Analog can be written for the heat pump to the heating QH:

In the heat pump heating capacity is the sum of the heat absorbed at ambient conditions QU and the technical work, such that:

.

The Carnot process represents the limiting case of a reversible process, which calls for ideal conditions that are not technically achievable.

The amount of heat can be expressed with the entropy S:

If the real process with the Carnot process is compared, can be written for refrigeration systems:

All temperatures T in Kelvin.

The change in entropy? S is identical for the reversible Carnot process for the two isothermal changes of state at the temperatures TH and TU and can thus be shortened.

Analogously, for the heat pump:

.

The performance figures technically realized compression refrigeration systems are usually about 1 in the area of ​​climate control with less difference between the temperature of the cooled air and the ambient performance numbers are accessible to 7.