DTLarca

15-02-2013, 12:14 AM

I'm looking to convert all my lectures to little books containing only my raw lecture notes and illustrations.

One of the topics is inspired by a section in John Tomczyk's good little book going by the same title as this post.

There are a small number of statements in his book that, when read my way, seem mistaken in a way that could invoke great discussion.

For instance, on page 11, up on the top left of the page, he states:

"This additional superheat decreases the density of the refrigerant vapour to prevent compressor overload, resulting in lower amp draws".

However, we can calculate the power required to compress a volume of gas, from one condition to another, using a formula that does not have density or mass amongst its variables - for instance:

W = PV y/(y-1) [(P2/P1)^{(y-1)/1} - 1]

Where y = Cp/Cv

This means that if the spoken-of reduced density does not occur on account of a reducing pressure then there will be no change in amp draw - and in fact - when you see on any PH chart that the slopes of constant entropy increase, in the direction of increased enthalpy, with increased superheat, we can rightfully expect amps to increase per kg pumped if only somehow mass flow could at the same time be at least kept up.

Then, for a fixed head pressure, as suction pressures rise, there arrives a point where motor amps peak and thereafter begin to drop-off again.

So, at first glance, it seems to me that John may very well be in error here.

One of the topics is inspired by a section in John Tomczyk's good little book going by the same title as this post.

There are a small number of statements in his book that, when read my way, seem mistaken in a way that could invoke great discussion.

For instance, on page 11, up on the top left of the page, he states:

"This additional superheat decreases the density of the refrigerant vapour to prevent compressor overload, resulting in lower amp draws".

However, we can calculate the power required to compress a volume of gas, from one condition to another, using a formula that does not have density or mass amongst its variables - for instance:

W = PV y/(y-1) [(P2/P1)^{(y-1)/1} - 1]

Where y = Cp/Cv

This means that if the spoken-of reduced density does not occur on account of a reducing pressure then there will be no change in amp draw - and in fact - when you see on any PH chart that the slopes of constant entropy increase, in the direction of increased enthalpy, with increased superheat, we can rightfully expect amps to increase per kg pumped if only somehow mass flow could at the same time be at least kept up.

Then, for a fixed head pressure, as suction pressures rise, there arrives a point where motor amps peak and thereafter begin to drop-off again.

So, at first glance, it seems to me that John may very well be in error here.