York YMC2 A, Operation And Maintenance

JOHNSON CONTROLS

19

Section 3 - Maintenance

Form 160.78-O1 

Issue date: 05/19/2021

3

the shells. If a source of hot water is not readily 

available, use a portable water heater. 

Do not use 

steam.

Connect a hose between the source of hot water 

under pressure and the evaporator head drain con-

nection, out the evaporator vent connection, into 

the condenser head drain, and out the condenser 

vent. To avoid the possibility of causing leaks, 

raise the temperature slowly so that the tubes and 

shell are heated evenly.

5.  Close the system charging valve and the stop 

valve between the vacuum indicator and the vac-

uum pump. Then disconnect the vacuum pump 

leaving the vacuum indicator in place.

6.  Hold the vacuum obtained in 

Step 3

 above in the 

system for 8 hours; the slightest rise in pressure 

indicates a leak or the presence of moisture, or 

both. If after 24 hours the wet bulb temperature 

in the vacuum indicator has not risen above 40°F 

(4.4°C) or a pressure of 6.3 mm Hg, the system 

may be considered tight.

Be sure the vacuum indicator is valved off 

while holding the system vacuum and be 

sure to open the valve between the vacuum 

indicator and the system when checking 

the vacuum after the 8 hour period.

7.  If the vacuum does not hold for 8 hours within the 

limits specified in 

Step 6

, the leak must be found 

and repaired.

Vacuum dehydration

To obtain a sufficiently dry system, use the following 

procedure to evacuate and dehydrate a system in the 

field. Although there are several methods of dehydrat-

ing a system, this method produces some of the best 

results and provides accurate readings as to the extent 

of dehydration.
The equipment required to follow this method of de-

hydration consists of a wet bulb indicator or vacuum 

gauge, a chart showing the relation between dew point 

temperature and pressure in inches of mercury in a 

vacuum (see 

Table 1

), and a vacuum pump capable of 

pumping a suitable vacuum on the system.

Operation

A refrigerant system can be dehydrated using this 

method because the water present in the system reacts 

to changes in a similar way that a refrigerant does. 

By pulling down the pressure in the system to a point 

where its saturation temperature is considerably be-

low room temperature, heat will flow from the room 

through the walls of the system and vaporize the water, 

allowing a large percentage of it to be removed by the 

vacuum pump. The length of time necessary for the de-

hydration of a system depends on the size or volume of 

the system, the capacity and efficiency of the vacuum 

pump, the room temperature and the quantity of water 

present in the system. By the use of the vacuum indi-

cator as suggested, the test tube will be evacuated to 

the same pressure as the system, and the distilled water 

will be maintained at the same saturation temperature 

as any free water in the system, and this temperature 

can be observed on the thermometer.
If the system has been pressure tested and found to be 

tight before evacuation, then the saturation temperature 

recordings should follow a curve similar to the satura-

tion curve shown in 

Figure 6

.

The temperature of the water in the test tube will drop 

as the pressure decreases, until the boiling point is 

reached, at which point the temperature will level off 

and remain at this level until all of the water in the 

shell is vaporized. When this final vaporization has 

taken place the pressure and temperature will continue 

to drop until eventually a temperature of 35°F (1.6°C) 

or a pressure of 5 mm Hg is reached.

LD00474

Figure 6 - 

Saturation curve