Lead – Lag

 Have you heard of the wonderful one-hoss shay,

That was built in such a logical way

It ran a hundred years to a day,

And then, of a sudden, it — ah, but stay,

I’ll tell you what happened without delay

Oliver Wendell Holmes (1809-1894)

 Most multi compressor refrigeration plants are built with a control function that allows either manual or automatic selection of a lead or “first on” compressor. Most operators, maintenance managers, and designers will agree that this function is often to permit equal running time for each compressor or pump or other piece of equipment.     

I ask “why?”

For plants with different size compressors, there is often a benefit to being able to choose a lead machine based on seasonal load variations but for plants with equal or almost equal sized machines, equalizing running time only results in uneven maintenance costs over time and reduced reliability.

Equalizing operating hours means that all of your equipment wears out at the same time. It also means that all of your equipment needs periodic maintenance at the same time. The effect is;

-        Low maintenance bills some years and a major bill other years

-        Risk of having to shut down the system for maintenance since every item needs maintenance at almost the same time.

-        Risk of unplanned system shutdown since every item is equally worn out.

 Lead – Lag switching should almost never be practiced. The benefits of almost always using the same compressor or pump as the first on machine (lead) are;

-        Only one unit needs maintenance more often, others will need major maintenance after longer periods. This will help level the maintenance budget each year.

-        There is always at least one unit in top running condition due to low operating hours.

-        In the longer term, one unit only will need to be replaced at the end of it’s normal life (years). Other pieces will be replaced in later years.

-        Risk of planned or unplanned shutdown is greatly reduced.