Evaporator units transfer a working fluid from its liquid state into the vapor state. Such units are needed, for example, in space craft and space stations for discharging of waste heat. Such evaporator units have at least one evaporating element in which a liquid containing space is separated from a vapor or steam containing space by a separation element that is constructed as a capillary structure. These evaporators are known as so-called capillary evaporators. Capillary evaporators are intended, especially for use in space flight applications, for example, in space platforms and reusable transport systems such as a space shuttle system.
Waste heat is generated in all space flight bodies due to the energy conversion resulting from the propulsion, the control, the signal processing, and due to experiments as well as due to metabolic processes. The generation of waste heat cannot be avoided due to the limited thermo-dynamic efficiency of the systems involved. The so generated waste heat must be discharged in such a manner that an undesirable heat-up of the space craft or space station is avoided. For accomplishing this purpose it is necessary to transport the waste heat within the space craft or station from a point where the waste heat is generated to a point suitable for discharging the waste heat into the environment of the space craft or station. The transport of the waste heat between its point of generation and a distant heat sink by means of a closed fluid circulating circuit and the direct discharge of heat by an evaporating working fluid enable the desired temperature control. The first type of system in which heat is transported from its source to a heat sink, are is referred to as so-called transport systems. The second type of system in which heat is discharged by the evaporation of a working fluid, is referred to as so-called discharge systems. The transport systems are primarily useful for larger orbital stations. The discharge systems are primarily useful in so-called "space transports" such as a space shuttle.
Both systems have a common feature in that they require an evaporator unit for the heat discharge. The trouble-free proper operation of such an evaporator unit requires that two conditions are satisfied. The first condition requires that the heated surfaces are wetted at all times with the working fluid to be evaporated, for example water or ammonia. The second condition requires that the liquid phase is effectively separated from the vapor phase in the evaporator unit.
The above conditions are satisfied in evaporators working on earth, by the gravity effect under which such an evaporator is working. However, the effects of gravity cannot be relied on when such evaporators are used in outer space where an external acceleration force is either entirely absent, such as in an orbital station, or such acceleration force changes in its size and effective direction as is the case in a space transport or space shuttle system. For this reason the above mentioned capillary evaporators have been used in space craft as evaporator units forming part of the evaporation systems. These evaporator units make use of the capillary principle in which capillary elements function as a quasi-wick. Due to the effect of the wick, the liquid and the vapor are separated from each other in such evaporator units. The capillary structure for this purpose is part of a so-called separator element and the heat is conducted through a special arrangement of ribs through the vapor space to the vapor facing surface of the capillary structure functioning as a wick. Due to the capillary forces the wick is saturated with the working fluid at all times. These capillary forces simultaneously make sure that the mass flow of the working fluid necessary for the cooling effect, adjusts itself automatically.