CONTROL & PROTECTION – SYSTEM OVERVIEW

The Master Class series of articles have covered a wide range of topics, some related to specialised applications and some to standard components.  However, in some instances, the principles behind their selection has not been covered.  Therefore, before proceeding further into Capacity Control and the specific aspects of refrigeration and air conditioning, we will now look at a complete system to summarise the content of certain preceding articles.

Most of the components discussed so far are shown in the drawing shown in Figure 1.

Control Systems

Control systems can be classified into three categories:

1. Controls necessary for the correct operation of the system
2. Controls that make a system safe
3. Controls that ensure the correct design conditions within the conditioned space or products are achieved

An examination of Figure 1 shows a typical refrigeration plant that consists of an air-cooled, multi-fan condenser connected to two compressors operating in parallel, which in turn are connected to three evaporators.  Each of the evaporators are connected to an individual Thermostat which in turn is connected to a Solenoid Shut-off Valve fitted in the liquid line feed to each of the Thermostatic Expansion Valves.

Each compressor is fitted with a Differential Pressure Oil Safety Switch that is subjected to the discharge pressure generated by the compressor mechanical oil pump and the pressure within the compressor crankcase at the suction inlet of the pump.

The system is fitted with a Safety High Pressure Cut-Out Switch, Low Pressure Cut-Out Switch and a Control Suction Pressure Switch.

All the major system components are fitted with Hand Shut-off Valves to enable them to be isolated.  Any components which can be isolated with refrigerant trapped inside are fitted with pressure relief valves. The major refrigerant containing components vent their safety relief valves to atmosphere while components containing a smaller refrigerant charge are arranged to relieve to the system low side. The low side can ventilate to atmosphere also if necessary.

The system incorporates an Evaporator Pressure Regulator and a Suction Line Accumulator.  A Capacity Regulating Valve and Liquid Injection Valve are also included.

If we follow the flow of the refrigerant from the receiver through the system we will encounter the three categories of control referred to earlier.

The first component in the flow path is a Shut-off Valve.  This component is frequently selected on the basis of the refrigerant line size but there are many types of Shut-off Valve ranging from Ball Valves to Globe Valves each with its own characteristics which include the pressure drop through the valve, positive shut off, spindle and gland gas tightness.  Some of these characteristics change according to whether the valve is fully open or fully closed.

Generally a Ball Valve is recommended for fitting as a normally open valve. It has little or no pressure drop if it is selected on the basis of its bore matching the pipe line.  It is therefore particularly suitable for suction lines.

Where positive shut off is a primary requirement, then Globe Valves or “Y” type Shut-off Valves may be more suitable than Ball valves.

Hand Shut-Off Valves that are normally fitted for maintenance purposes only and are not used during normal operation of the plant should be fitted with a vapour tight cap.  Not only does this solve the problem of potential refrigerant leakage through the spindle gland, it also deters unauthorised personnel from changing the valve position.

The valve caps must comply with BSEN.378 in that they must be able to relieve any pressure that may have built up under the cap, due to leakage, in a safe and controlled manner. This usually consists of a small hole drilled into the threaded section of the cap and as the cap is unscrewed the hole is exposed. This feature would also reveal whether or not the spindle gland is intact and help prevent the removal of the cap which could result in a significant vapour leak.

The next component encountered in the flow is the Refrigerant Drier, a component which has increased in importance due to the introduction of the new range of refrigerants and their associated requirement for synthetic-ester oils.  These oils are particularly hygroscopic and if incorrectly handled can introduce significant amounts of moisture into the system.

Drier Selection

Ensure drying medium is suitable for the refrigerant in the system.

Note: Some manufacturers have reservations about the use of driers containing Activated Aluminium with systems containing HFC refrigerants.

Ensure drying medium is suitable for the type of oil in the system.

Select drier on the basis of system duty and volume of system charge⁽ⁱ⁾.

Check pressure drop through drier at design duty⁽ⁱⁱ⁾.

 ⁽ⁱ⁾ Direct expansion systems that operate with a near constant load, or use high-side float control will have a relatively small refrigerant charge for a given duty.  Systems that have large load variations or incorporate flooded evaporators or are of the pumped circulation type will have a large refrigerant charge for a given duty.

The latter type of system should have the largest drier core capacity for its specified duty.

The next component is a Pressure Relief Valve.  If the liquid line contains liquid refrigerant which is sub-cooled below ambient temperature and if it is also possible to trap this liquid between two isolating devices (Shut-off Valves, Solenoid Shut-off Valves) then when the liquid warms up, it will expand.  As there will then be insufficient space for expansion, the refrigerant will rupture the weakest point, in this case the drier assembly. BSEN.378 therefore recommends that a Pressure Relief Valve be fitted in order to protect the assembly from rupturing with potential for personal injury.

Pressure Relief Valves, as opposed to other forms of pressure safety devices such as Bursting Disks and Fusible Plugs, have the ability to reseal themselves once the excess pressure has been discharged. The re-sealability of a Pressure Relief Valve permits discharge from one side of a component to an area having a lower pressure and the ability of the valve to reseal itself prevents any possibility of reverse flow taking place, or indeed continuous by-pass to then occur.

This is illustrated in Figure 1.

The next component is the Liquid to Vapour Heat Exchanger, sometimes referred to as a Suction-Liquid Interchanger.  The primary purpose of this component is to sub-cool the liquid refrigerant thereby improving system cooling capacity without power input penalty.  As a consequence of sub-cooling the liquid, the suction refrigerant vapour is super-heated and this reduces the amount of liquid entrained in the vapour. This latter feature can be useful when the suction lines are short as this will reduce sweating at the compressor.

To calculate the amount of heat transferred from the liquid side to the suction vapour the formula;

Q = k x A x  t_m          Where Q = Heat flow – Watts

1. = heat transfer coefficient – W/m²°C.

A = Transfer area of heat exchanger – m²

t_m = Average temperature difference – K.

 

t_m = (t_max – t_min ) / ln (t_max / t_min )

The Liquid to Vapour Heat Exchanger must not be oversized since excessive superheating of the refrigerant suction vapour will increase specific volume thereby reducing mass flow through the compressor.  Inadequate cooling of the compressor motor in the case of semi-hermetic compressors would also result.

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.