Alternative secondary fluids for indirect refrigeration systems
The secondary fluid is a complex mixture which also contains pH adjusting agents, antioxidants, anti-foaming agents, stabilizers, colorants, etc. These additives can stabilize and improve the fluid performance but also affects the thermophysical properties. Secondary fluid is water-based solutions of organic or inorganic substances (such as alcohols, glycols and salts) used in various indirect refrigeration systems and heat pumps. The advantages of the indirect refrigeration systems are: low refrigerant charge, low risk of refrigerant leakage, the fact that the primary refrigerant system is localized in the machine room, easy and precise control, no oil recovery problems, refrigerants like ammonia and hydrocarbons can be used; easy maintenance, no technical limitations on the distance between refrigeration and machine room, plastic pipes can be used and that it will be longer intervals between defrost periods when air cooling due to more uniform frost formation.
Background
Unfortunately, many of the currently used secondary fluids have corrosive nature but it is possible to minimize corrosion risks by using the right inhibitors. All additives stabilize and improve performance in some respects, but affect the secondary fluid’s thermophysical properties in a way that is not well known. Additionally, new type of adsorption corrosion inhibitors belonging to the so-called "green corrosion inhibitors group" that are in 100% organic/plant based. Plants are sources of naturally occurring substances with complex molecular structures that have different chemical, biological and physical properties. These naturally occurring substances are used because they are environmentally acceptable, cost-effective and are easy to find in nature. These advantages are the reason for the use of extracts of some plants and products as corrosion inhibitors for metals. Various plant extracts that can be used as the corrosion inhibitors are generally called green corrosion inhibitors.
Ethanol based aqueous solutions are commonly used secondary fluids in the geothermal heat pumps in Sweden. The available ethanol based commercial products contain up to 10% denaturating agents and no corrosion inhibitors. Measurements recently performed at KTH shows that the presence of these additives in ethanol enhanced the thermophysical properties, in particular the specific heat capacity, thermal conductivity and dynamic viscosity. So far, ethanol based refrigerants have not had corrosion inhibitors. Further studies are needed to find suitable corrosion inhibitors that can be used instead of or together with denaturation products to effectively protect the system against corrosion.
Alternative refrigerants will be explored: environmentally friendly organic synthetic diesel oils and ionic liquids. The first results show that these diesel oils have a low dynamic viscosity at low temperatures and have wider operating temperature range compared with aqueous solutions of glycols and also silicone oils. Synthetic diesel oils can be an interesting alternative to expensive silicone oils as the heat transfer fluid at low temperatures. Advantages of diesel oils are non-corrosive nature, a relatively high flash point and lubrication properties. Ionic liquids are fluids composed of positive and negative ions, such as salts. Ionic liquids characterized in that they have low vapor pressure and therefore are very difficult to distill. They are usually not flammable, are good conductors of electricity and are often difficult to oxidize and reduce. Some types of complex organic salts of formate and acetate (having for example, amino groups) will be tested.
Objectives
1. Compile a report about the thermophysical properties of various secondary fluids with corrosion inhibitors, alternative secondare fluidss, and ethanol mixtures with potential corrosion inhibitors.
2. Validate the properties of ethanol-isopropanol mixtures using measurements of thermophysical properties. Identify and test potential adsorption corrosion inhibitors compatible with ethanol.
3. Test the thermophysical properties of various synthetic diesel oils and organic salts for applications at both low and high temperature.
4. Validate the effect on heat transfer process of corrosion inhibitors that form physical barriers.
5. Validate the effect on the heat transfer process by the adsorption corrosion inhibitors.