Pressure controls

Tuesday, March 1, 2011

The actual pressure in the system at the point monitored by the pressure control is usually provided via a branch line or – if the pressure control is installed directly on a line – applied directly to the pressostat. The pressure actuates a potential-free set of contacts that can be used to control a device such as a compressor or a fan.

Chapter 1. Pressure controls

In addition to pressure regulators, which were described in the previous issue, refrigeration systems have pressure controls. Pressure regulators directly affect the main mass flow of the refrigeration circuit by opening and closing, but pressure controls (also called pressostats) have a different function. 

In addition to pressure regulators, which were described in the previous issue, refrigeration systems have pressure controls. Pressure regulators directly affect the main mass flow of the refrigeration circuit by opening and closing, but pressure controls (also called pressostats) have a different function. The actual pressure in the system at the point monitored by the pressure control is usually provided via a branch line or - if the pressure control is installed directly on a line - applied directly to the pressostat.

The pressure actuates a potential-free set of contacts that can be used to control a device such as a compressor or a fan.

Chapter 2. The job of a pressure control

What are these controls good for, and why are they necessary? Let's start with the function of a low-pressure control in a compressor refrigeration system. Low-pressure controls, which are usually fitted close to the compressor on the suction side, are most often used to protect the compressor by switching it off if there is not enough refrigerant. The reason for this is that a loss of refrigerant in the system can lead to a variety of problems. 

For example, suction gas cooling is reduced, which especially with compressors that rely entire on suction gas cooling is a major problem. In addition, compressor oil always escapes in addition to the refrigerant when there is a refrigerant leak because it is constantly passing through the entire refrigeration circuit along with the refrigerant. This can lead to insufficient lubrication. It is thus very beneficial to switch off the compressor as a precautionary measure if the low pressure level drops sharply, in order to avoid compressor damage or total failure of the compressor. 

It should be noted that the low-pressure control can also shut off the compressor for reasons other than a shortage of refrigerant. If a problem with low-pressure control-off occurs, the service fitter should always check the volume flow rate of the media cooled in the evaporator. If it is too low due to some other malfunction, there is probably not a shortage of refrigerant. A simple example here would be a defective evaporator fan or a defective pump with a water chiller unit (refrigerant/water evaporator). In the latter case, however, the flow switch in the water circuit should have also tripped, as otherwise there would be a risk of evaporator icing (and breaking).

Chapter 3. Pump-down and pump-out circuits

A low-pressure control can also be used for control purposes. For example, it is used in pump-down and pump-out circuits (which are commonly used in refrigeration systems) to switch off the system when the compressor is switched off for control reasons (such as when the switch-off temperature of the thermostat is reached). 

This occurs when the pressure resulting from closing the liquid solenoid valve while the compressor continues to run has been sucked down from the liquid solenoid valve through the evaporator all the way to the suction side. When a certain threshold value is passed, the low-pressure control shuts off the system. When the switch-on temperature of the room thermostat has been reached again after a while and the solenoid valve opens, the pressure in the suction line rises and the low-pressure control starts the compressor again when the pressure exceeds its switch-on point.

Chapter 4. High-pressure safety controls (DWK - DBK - SDBK)

One application for high-pressure controls is the high-pressure protection. The EN 378 standard specifies that a high-pressure control must always be fitted in a commercial refrigeration system. Its purpose is to switch off the compressor, which is normally the primary source of pressure in a compressor refrigeration system. If the allowable operating pressure has been reached and an overpressure situation arises, the compressor is switched off by the high-pressure control and the pressure on the high-pressure side decreases.

High-pressure safety controls for this purpose are classified into three types: with automatic reset (DWK), with manual reset by hand (DBK) and by tool (SDBK). With an automatic reset (DWK) version, it is always possible for the system to be switched on again automatically after the pressure decreases by a certain amount. With a manual reset by hand (DBK) KP, a manual reset must be performed after a protective switch-off. It must be possible to initiate this reset by hand without using a tool. Finally, with a manual reset by tool (SDBK) device a tool is necessary to perform a manual reset.

Chapter 5. Condenser fan control

Another possibility on the high-pressure side is to use a pressure control to control the condenser fan. This provides an economical alternative to a variable-speed fan control for ensuring that the condensing pressure does not drop too low, especially in cold weather. This option is especially attractive for very small commercial refrigeration systems, which are naturally quite cost-sensitive, as well as for larger systems with three-phase fans for which a variable-speed controller can be very costly.

Chapter 6. Types of construction

There are basically two types of construction for pressure controls. The first type is an adjustable standard pressure control for wall mounting, and the second type is a cartridge-type pressure control. Wall-mounted pressure controls (such as the Danfoss KP) are especially popular with fitters. They have an adjustable switching pressure, and the tubing does not have to support their entire weight. In addition, it is possible to fit the pressure control remotely in a machine cabinet at the front of the system, which can distinctly increase accessibility and ease of servicing. By contrast, a cartridge-type pressure control is the solution preferred by mass-production manufacturers because its fixed setting cannot easily be modified in the field by unauthorized persons. In addition, cartridge-type pressure controls are usually very inexpensive. 

Contact rating 

An important consideration when using pressure controls with potential-free contacts is the contact rating. This can be confusing at first because manufacturers generally specify three different contact ratings. Which value should you be guided by? 

The three values are normally given for the following cases: a pure ohm load (the highest load is traditionally possible in this case), a partially inductive load, and a purely inductive load. An example of a pure ohm load (load designation: AC 1) is an electrical (resistance) heater for defrosting. An example of a partially inductive load (AC3) is an electric motor (which also includes compressors). 

On the other hand, a coil (AC 15), such as the coil of a solenoid valve, acts as an inductive load on the contacts of a pressure control.

Chapter 7. Electrical connection

A standard pressure control with changeover contacts normally has three contacts to which the conductors of the electrical cable can be connected. The functions of these three contacts are 'phase in', 'fault', and 'phase out (to the motor)'. If only two contacts are wired, it does not make any difference which of the two phase contacts is connected to each conductor. 

The 'fault' contact is rarely used. It could possibly be used for a fault indication, such as driving a red signal lamp or sending a signal to a remote maintenance station. 

With the Danfoss KP1 (low-pressure control), the 'phase in' connection is contact 1 and the 'phase out' connection is contact 4. If it is desired to use the fault function, a connection can be made to contact 2 (please note that in case of pump down the fault contact is active). Here it is important to note the connections for the KP7 high-pressure control are not the same. Although the 'phase in' connection is still contact 1, the 'phase out' connection is contact 2, while contact 4 is for the fault signal. This may appear confusing, but it is easy to explain the logic of this for service fitters, using the KP pressure control as an example. After removing the plastic cover, you will see the set of contacts at the right. They are labelled '1', '2' and '4'. 

With a KP 1 low-pressure control, contact 4 is at the top and contact 2 is at the bottom. The switch-off condition occurs when the pressure drops (for example, due to a shortage of refrigerant). As the bellows of the pressure control always moves upward with increasing pressure (the bellows expands) and downward with decreasing pressure, switch-off must occur when it moves downward. This means that contacts 1 and 4 of a KP1 should be used for the connecting circuit between the AC mains and the compressor.

By contrast, with a KP7 high-pressure control the switch-off occurs when the bellows moves upward. As contact 4 is always at the top and contact 2 is always at the bottom, the conductors must be connected to contacts 1 and 2 in this case. In the case of a dual pressure control, which combines a high-pressure control and a low-pressure control (with the exception of the Danfoss KP7BS, which contains two high-pressure controls - DBK and SDBK), there are versions with a low-pressure fault contact or two fault contacts (high-pressure and low-pressure). Using a dual pressure control eliminates installation of a second electrical cable and a second mounting bracket. In addition, a dual pressostat is usually less expensive than two pressure controls.

Chapter 8. Refrigeration circuit connection

When connecting a wall-mounted pressure control to the refrigeration circuit, you should pay particular attention to the fact that high-pressure controls for safety functions (such as KP7W, KP7B, KP7S, and KP7BS) must always be connected using a branch line with an inside diameter of at least 4 mm. In other words, 6-mm copper tubing must be used. 

Of course, it is also possible to use a special plastic connection line for refrigeration systems, such as has become fashionable in the last 15 years, with an inside diameter of at least 4 mm.Pressure controls for condenser fan control and low-pressure controls can also be connected using capillary tubing, although many system builders always use 6-mm copper tubing or alternative plastic tubing for all pressure controls for the sake of appearance and to avoid unintentional blockage or pinching of capillary tubing.

Chapter 9. Manual testing

A manual test of switch operation should only be performed in exceptional cases. If such a test is absolutely necessary, it can be performed with a KP pressure control by inserting a screwdriver from the front side.Use the screwdriver to raise the steel plate attached directly to the bellows in order to simulate an upward motion of the bellows (increased pressure). This test should be performed with extreme care, and it should only be performed under exceptional circumstances.

Chapter 10. IP protection class

The IP protection class can also be an important consideration, depending on local circumstances and ambient conditions. The first digit of the IP protection class code (e.g. 'IP54') stands for the degree of protection against object penetration, while the second digit stands for the degree of protection against water. An IP class of IP4x means that it must not be possible for a wire with a diameter of 1 mm to penetrate the device. IPx4 means that the device is suitable for use with splashed water from all directions. 

Generally speaking, it can be said that the higher the protection class, the better the device is protected against dust, dirt particles and moisture. Standard Danfoss KP pressure controls have a protection class of IP33 with respect to the effects of dust and moisture if they are not fitted with a top cover. If the top cover provided with the control is fitted, the protection class is increased to IP44. A protection class of IP55 can be achieved by using an accessory protective housing. If an even higher IP protection class is desired, you can use the RT series, which is designed for especially harsh environments.The controls in this series have a protection class of 54 to 66, depending on the model.