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METHOD AND TRANSPORT PROPERTIES FOR ENHANCING THE NUCLEATIVE HEAT TRANSFER OF REFRIGERANT CHILLER EVAPORATORS

IP.com Disclosure Number: IPCOM000129135D
Original Publication Date: 2005-Sep-28
Included in the Prior Art Database: 2005-Sep-28
Document File: 5 page(s) / 34K

Publishing Venue

National Institute of Standards and Technology

Related People

Mark A. Kedzierski: INVENTOR

Abstract

A method for improving the efficiency and/or for increasing the capacity of a chiller1 by introducing a lubricant of specified transport properties to synergistically compliment the transport properties of the existing lubricant in the chiller is disclosed.

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METHOD AND TRANSPORT PROPERTIES FOR ENHANCING THE NUCLEATIVE HEAT TRANSFER OF REFRIGERANT CHILLER

EVAPORATORS

ABSTRACT

A method for improving the efficiency and/or for increasing the capacity of a chiller1 by introducing a lubricant of specified transport properties to synergistically compliment the transport properties of the existing lubricant in the chiller is disclosed.

SUMMARY OF THE INVENTION

It is the objective of this invention to describe the required surface-tension, viscosity, composition, and chemical characteristics of a lubricant that will enhance the nucleate boiling heat transfer of a refrigerant/lubricant mixture when added in dilute quantities to the mixture. The required transport properties of the added lubricant (additive) are relative to those of the existing lubricant. The additive must have a greater surface- tension and a greater viscosity than the existing chiller lubricant and it must also be somewhat "chemically dissimilar" to the existing chiller lubricant. The greater surface- tension and the "chemical dissimilarity" work together to encourage the additive to form a mono-layer between the boiling surface and the remaining lubricant. Now that the additive is at the heat transfer surface, the greater viscosity of the additive acts to produce more nucleation, which in turn improves the boiling heat transfer as compared to the original refrigerant/lubricant mixture. The required amount of additive is dictated by the mass of active monolayer of additive that coats the evaporator surface and the mass of sacrificial additive that coats the remaining internal surfaces of the chiller.

DETAILED DESCRIPTION OF INVENTION

Formation of Additive Monolayer

The first necessary requirement for a lubricant additive to form a monolayer on a solid wall in a boiling liquid refrigerant/lubricant/additive system is that the additive must have a liquid-vapor surface tension that is greater than that of the lubricant. If the monolayer does not form, the additive is likely to remain well mixed with the lubricant. For this case, the mixed lubricant and additive form a single layer (excess surface density) at the boiling surface as shown for system 1 in Fig. 1. The formation of lubricant excess layers at boiling surfaces is well documented and measured in Kedzierski (2002). System 2 in Fig. 1 shows the case for which the additive does form a monolayer.

1Efficiency improvements are based on improvements in evaporator heat transfer performance. Heat transfer improvements do not necessarily guarantee improvements and/or changes in chiller performance because of other factors that influence HVAC equipment performance.

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If present, system 2 will evolve from system 1. The evolution can occur spontaneously only if the change from system 1 to system 2 results in a reduction of system surface energy (Rosen, 1978). The requirement for system 2 to exist can be expressed in terms of surface energies by applying the analysis of spr...