MASTERCLASS – AIR CONDITIONING TECHNOLOGY

This month we continue to look at compressor protection devices with the emphasis on the important topic of lubrication and its associated control devices for system protection.  We will also examine other aspects of system control which, if incorrectly applied, may have an adverse effect on the compressor lubrication system.

Pump Down Control

Pump down is a useful method of control which reduces the risk of liquid refrigerant flooding within the compressor on start-up.  This excess liquid can cause foaming of the oil, which reduces its viscosity and lubrication qualities.  It can enter the cylinders resulting in damage to bearings and valve plates due to hydraulic compression.

Liquid flood back can occur for the following reasons:

• Excessive amounts of liquid entering the evaporator during the off cycle whilst the compressor is at rest due to lack of cooling demand
• As a result of the thermostatic expansion valve phial being un-insulated and being warmed by ambient air
• The TEV valve seat failing to close fully.

As a consequence, when the compressor starts, a rush of liquid refrigerant (usually referred to as a “slug”) flows down the suction line and enters the compressor.

With pump down control, when the required setpoint temperature is reached, the system controller, usually a thermostat, will de-energise a solenoid shut-off valve located in the liquid line (Liquid Line Solenoid Valve).  This valve will close and since no more refrigerant can enter the evaporator, the compressor will continue to draw superheated vapour from the evaporator thus emptying the evaporator.  This is in turn results in a progressive fall in the suction pressure as a result of the diminishing amount of vapour available at the compressor inlet.  A low pressure switch will then switch off the compressor when the cut-out setpoint is reached.

When the temperature in the conditioned space rises above the setpoint of the thermostat, the liquid line solenoid valve is energised and the valve opens.  Refrigerant passes into the evaporator and the suction pressure rises in accordance with the saturation temperature of the refrigerant within the system.  When the pressure exceeds the low pressure switch setting (plus differential), the switch closes contacts and starts the compressor.  Since there is no excess refrigerant in the evaporator, (as can be the case on systems without a pump down facility), the compressor experiences a greater degree of protection by starting in a relatively unloaded condition and without the possibility of liquid slugging.

In order to reduce the flow of refrigerant into the evaporator after the liquid line solenoid valve closes, it is advisable to fit the valve as close as is practical to the expansion valve.

If the solenoid valve is fitted close to the condenser / receiver on a system with a long liquid line, the pump down sequence can be protracted and may even result in a number of short cycles under the control of the low pressure switch.

Some thought must be given to the low pressure switch settings.  It is not uncommon for the Service Engineer to set the safety low pressure switch to cut-out at 0.136 bar (2 psig) on the basis that the system will be protected from running into vacuum conditions in the event of a leak.  However a typical air conditioning system evaporates at 5°C.  If the refrigerant in the system is HCFC22, this equates to a saturated pressure of  5 bar (69 psig).   Therefore, in order to cut-out on the low pressure switch, the saturation temperature will be the equivalent of minus 38°C when the pressure falls to 0.136 bar (2 psig).  This sudden reduction in pressure to 0.136 bar (2 psig) on a system which has been running at a balanced condition of 5 bar (69 psig) causes the refrigerant mixed within the crankcase oil to evaporate which subsequently leads to oil foaming.  Foaming oil is not a very good lubricant and is also easily pumped out of the compressor.

Therefore when setting the low pressure switch on a pump down system, the cut-out point should really be set 0.2 – 0.27 bar (3 – 4 psig) lower than the normal operating pressure.  However, one must not forget the conditions that may exist if capacity control is incorporated.

Although we have referred to Low Pressure Switches (LPS) and Thermostats we have not yet covered the construction details.

A LPS is connected by a capillary line or a small bore tube copper tube to the low pressure suction side of a compressor.  The suction pressure passes through the connecting tube into a bellows assembly which expands as the pressure increases and contracts as the pressure falls. The movement of the bellows acts upon a pivoting lever which incorporates a snap action, controlled by a spring.  Most LPS are fitted with a single pole, double throw switch mechanism and can be wired so that they can electrically make or break on fall of pressure.

The most common arrangement is for the switch to break contact on fall of pressure and the opening point is called the cut-out point.  The cut-in point or contact make point, is determined by the differential pressure setting.  The typical range of cut-out pressures is between 0.5 bar(a) and 8.0 bar(a).  The differential pressure range is between 0.3 bar and 4 bar.

To set an LPS to cut out at say 4 bar and in at 4.75 bar, identify the range adjusting screw and wind the screw until the pointer indicates 4 bar (Note – this is an approximation only),  pressurise the switch (with dry nitrogen) until the contact makes.  Then bleed the pressure slowly and read the pressure level at which contact breaks.  Adjust the switch accordingly and repeat the operation until the switch consistently breaks at the required pressure.  Now identify the differential adjusting screw and adjust the pointer to 0.75 bar.  (FIG 1)

Reduce the pressure until the switch breaks then gradually bleed the pressure into the switch and note the pressure at which the contacts make.  Adjust the differential accordingly and repeat the procedure until the switch operates consistently at the desired pressure.

It is essential that the cut out pressure is set correctly first, followed by the differential pressure.  LPS cut out and differential scales are always marked on the switch.

LPS’s are available with auto-reset or manual reset.  They can also be supplied with integral indicator lamps to show when the contacts are closed and we certainly advocate their use as a time saving device when undertaking diagnostic work on systems.

High Pressure Switches

High Pressure Switches (HPS) are in every way identical with LPS’s in terms of construction and appearance, the only difference being the rating of the bellows assembly.  The HPS is fitted with single pole double throw switches and can be wired to make or break on rise of pressure.  HPS’s are available in ranges between 3 bar and 30 bar, with differential pressure setting ranges between 2.1 bar and 8 bar.

When setting an HPS, set the opening pressure by adjusting the range screw, then set the closing pressure by adjusting differential screw.

Note:  In order to achieve the high pressures required for testing the cut out and differential settings, it is essential that this is achieved using dry nitrogen and a pressure reducing valve as opposed to blocking the condenser coil airflow which could lead to system damage or pipework fracture and subsequent injury.  The practice of closing the discharge valve while the compressor is running to pressurise the HPS is exceedingly dangerous and must not be attempted.

Thermostats

Thermostats are available in many forms, including the following:

• electronic
• electro-mechanical
• fluid sensing
• air sensing
• surface contact sensing

The electro-mechanical thermostat operates in a very similar manner to pressure switches but instead of being connected to a remote pressure source, the thermostat bellows is connected to a sealed phial containing a small liquid charge of refrigerant similar to Thermostatic Expansion Valves covered earlier on the series.  The pressure exerted by this refrigerant increases with a rise of temperature and decreases with a fall of temperature in accordance with the saturation temperature-pressure relationship.  Accordingly, the bellows activates a switch mechanism which can result in make of electrical contact on rise of temperature thereby starting a cooling cycle or make on fall of temperature, initiating a heating cycle.

The thermostat is set up in a similar manner to a pressure switch.  If the switch makes on rise of temperature, the range screw is adjusted until the pointer shows the cut-in temperature and the cut out temperature is set by the differential screw.  Conversely, if the thermostat makes on fall of temperature, the cut in point is set by the range screw and the cut out point is controlled by the differential adjustment.

Attention should be paid to the location of a thermostat or, more importantly, the thermostat sensing phial.  The sensor should always be positioned so that it subject to air movement and should not be located in a pocket of still, dormant air.  Ideally the sensor will be located in the return air stream to the evaporator where it will read the current temperature of the air in the conditioned space or system.

On close control systems where temperature recorders are fitted, the thermostat sensor should be located immediately adjacent to the recorder sensor to avoid misleading readings of true conditions.  Conversely it is unreasonable to measure the temperature in a room by randomly locating a thermometer or recorder without any reference to the temperature controlling sensor.

An especially important consideration is the temperature gradient that exists in all conditioned spaces.  The temperature controller can only measure the local conditions immediately surrounding it. (FIG 2)

Temperature gradient

The temperature gradient is the difference between air at the inlet diffuser discharging air into a room and the temperature of the air leaving the room at the return air grilles. This gradient is a function of the gains/losses in a room and the airflow rate / number of air changes per hour of conditioned air.  Close control systems will have a high number of air changes per hour while comfort conditioned rooms should be significantly less.

DISCLAIMER:  Whilst every effort is made to ensure absolute accuracy, Business Edge Ltd. will not accept any responsibility or liability for direct or indirect losses arising from the use of the data contained in this series of articles.