This invention relates to a system which monitors the viscosity of the lubricant in a compressor and takes corrective action should that viscosity fall below a desired level.
Compressors as typically utilized to compress a refrigerant such as in an air conditioning system are typically sealed in a housing. A suction refrigerant passing to the compressor will often pass within the interior of the housing and over the compressor motor through a suction port in a compressor pump unit. The refrigerant is compressed and driven through an outlet port to a downstream location such as a condenser. Compressors are often provided with a passage which selectively connects the discharge passage back to the suction passage. A valve typically closes the connecting passage, but may be selectively opened under certain system conditions. This valve is typically known as an unloader valve.
A motor is typically housed within the sealed housing, and drives the compressor pump unit. A series of bearings supports a shaft driven by the motor to drive the compressor pump unit. These bearings are typically provided with a lubricant which is received in a sump in the housing, and which is driven throughout the housing during operation of the compressor. The lubricant serves to cool and lubricate the bearings.
As system conditions change, the viscosity of the lubricant can change. In particular, as the lubricant heats its viscosity will change. Moreover, the necessary or minimum viscosity which would be desirable at the bearings will also vary as the operating conditions of the compressor change. As an example, should the speed of the motor or the load on the compressor pump unit increase, a desired minimum viscosity of lubricant will also change. In the prior art, the viscosity of the lubricating oil has sometimes become too low to adequately lubricate the bearings. Bearing damage and subsequent failure has sometimes resulted.
Another factor effecting the viscosity of the lubricant is that in the basic type of compressor described above, refrigerant also circulates with the lubricating oil. The oil can sometimes be diluted by liquid refrigerant, which can also lower the viscosity of the mixture.
The viscosity relates to a minimum oil thickness at the bearings. The compressor bearings, which are typically journal bearings, depend on a hydro-dynamic oil film to prevent metal-to-metal contact. The necessary oil film thickness is dependent on a number of factors including the dimension of the bearings, the speed of the shaft rotation, the viscosity of the oil and the load on the bearing. The several variables which interact as described above have sometimes resulted in the viscosity of the oil being insufficient to adequately protect a bearing. The present invention is directed to addressing the situation when the viscosity of the lubricant in a sealed compressor becomes too low.
In the disclosed embodiment of this invention, a control monitors the viscosity of the oil. The control is provided with a minimum viscosity for the particular compressor. If the detected viscosity drops below the minimum required viscosity, some corrective action is taken by the control. In a preferred embodiment, an unloader valve is opened. When the unloader valve is opened, the load on the compressor significantly decreases. This thus reduces the required viscosity and reduces the likelihood of any bearing damage due to the low viscosity. Also, unloaded operation may allow the viscosity to increase.
In a method according to the present invention, the viscosity of the oil in a compressor is periodically measured. The measured viscosity is compared to a minimum viscosity value. If the detected viscosity is above the minimum value, sensing simply continues. If however the viscosity is below a safe limit, then a corrective action is taken. While the corrective action can be as simple as stopping operation of the motor, in a preferred embodiment an unloader valve is opened. After the unloader valve is opened, the viscosity continues to be measured. Once the viscosity again increases above a safe limit, the unloader valve may be closed and the system can return to normal monitoring operation.
In a most preferred embodiment of this invention, the control also monitors aspects of the operation of the compressor such as the speed, etc. to define the minimum viscosity value. Moreover, the controller will typically be designed for each individual compressor such that the controller and its minimum viscosity values take into account the specific geometry etc. of the bearings utilized in the particular compressor.
These and other features of the present invention can be best understood from the following specification and drawings, the following of which is a brief description.