Pretty Hard Question about calculating a vacuum.....help

Hello

This is the final point of call before I give up on this question!!!

In one of our college assignments, we have a question which I really can't answer, and everyone in my class is struggling with. The lecturers wont even help, they just give us hints. To cut a long story short, i'll start with the main information, then follow it with the question:

MAIN SPECIFICATION

"After a long stand-by period, the refrigeration system of a medium sized cold store is assumed to be contaminated. Your service manager has sent you along to dry out the system, to check that it is not leaking, and then to correctly charge the system and leave it running. The system consists of a semi-hermetic condensing unit, which is located in a plant-room some 10 metres from the cold store in which is located a forced draught, finned cooler. The cooler is approximately 4 metres above the condensing unit, and the refrigerant enters it through an externally equalised expansion valve connected to the bottom of the cooler. The suction pipe leaves the top of the cooler. The ambient temperature is 10 degrees celcius, and both plant-room and cold store are at ambient temperature when you arrive on site."

QUESTION:

3a) Calculate a suitable vacuum necessary to ensure dryness at the stated ambient condition.

3b) Describe how to measure accurately the vacuum that you have identified as necessary in question 3a above

...now like I previously said, we are all struggling like hell with this, and the only hints our lecturers give us is "it has something to do with a steam table". I am aware that on steam tables it gives you total heat and specific volume of steam at corresponding gauge/absolute pressures, but regardless of that, I am dumbstruck. We was previously taught just a 24hr deep vac, or how to do a triple vac, not how to work out a 'suitable vacuum at the stated ambient'. I dont just want to read a figure off and write it down because thats not really the answer, the main part is how to work it out........................AAAAAHHHRRRR!!!!!!!!!!!

please help anyone

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I'm not going to do your work for you, but you had good advice the first time round and you need to combine this with best-practice refrigeration engineering, which your tutor should have taught you.

You need to obtain and use a set of absolute pressure steam tables - not steam tables for running high pressure steam.

From the tables, determine the vacuum pressure required to maintain a saturation temperature of water at 10 degrees. Next describe the method you would use to draw a vacuum on a large system while maintaining the desired pressure and why.

Thankfully, they don't seem to ask you what modifications you need to the system, how long this will take and how to demonstrate that the system is dry when you finish.

It might be a good idea to mention 'gas ballast'.......... but hope that they don't ask you what it is.


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