SEARCH

Masterclass:Externally Equalised Thermostatic Expansion Valves - Part 11

In part 10, capillary tubes, restrictor expansion devices and the thermostatic expansion valve (TEV) were studied. In this month’s article we continue our study of the range of expansion devices available.

Mike Creamer.


Construction

The construction of a typical thermostatic expansion valve can consist of the following: diaphragm housing, capillary and phial, which can be constructed from stainless steel and the valve body from brass. The capillary tube is often the most vulnerable part of the valve as it is often subject to vibration and flexing. The use of stainless steel would eliminate this weakness. The same philosophy can be applied to small pipes connecting statically-mounted HP and LP (high and low pressure) switches to vibrating parts of refrigeration circuits. This is particularly critical on large installations with a large refrigerant charge where such a fracture could lead to product stock losses and substantial release of refrigerant to atmosphere resulting in either stratospheric ozone depletion and/or global warming. The illustrations in Fig 2 below simplify the theoretical principles behind the internally and externally equalised thermostatic expansion valve:

Pb = bulb pressure (downward)

Pe = evaporating pressure (upward) (at evaporator inlet or outlet according to valve type)

Ps = spring pressure (upward)

Compressor protection

Thermostatic expansion valves maintain a given superheat in accordance with the spring pressure setting irrespective of evaporator pressure and loading. This key feature also has the disadvantage of allowing the compressor to become overloaded as a result of too high a pressure and entering gas temperature under peak load conditions. When the air conditioning system is first started, the temperature and therefore pressure in the bulb may be quite high. This causes the expansion valve to be partially or fully open when the compressor is started. This can lead to liquid flooding and damage to the compressor, particularly if the suction line is short or if the system is over-charged with refrigerant.

As the evaporating pressure and temperature drop, the expansion valve begins to regain control. Even under normal running conditions, the load at start-up may be substantial and this is particularly true of pull-down situations in refrigeration applications where the evaporator pressure and temperature are well above normally acceptable limits. Whilst it can be argued that a larger compressor will deal with this situation, a cheaper solution can be found in the selection of the correct expansion valve unless rapid pull-down to the desired temperature is essential.

Many expansion valves are fitted with a pressure-limiting feature (maximum operating pressure - MOP), which modulates the flow of refrigerant through the valve when the evaporating pressure rises above a pre-set level, irrespective of the bulb pressure and action of the diaphragm on the needle valve. This solves both of the aforementioned problems.

 

Figure 2: Theoretical principles behind the internally and externally equalised thermostatic expansion valve
 

 


Maximum operating pressure (MOP)

This defines the level of the pressure-limiting characteristic of the expansion valve. This pressure-limiting feature can be achieved by use of a spring positioned between the diaphragm and the needle valve stem, which opposes the action of the bulb. Certain types of valve allow adjustment of this spring pressure.

An alternative method utilises a collapsible chamber (cartridge), which is positioned between the diaphragm and the valve stem. The cartridge is filled with a non-condensable gas, which virtually remains at constant pressure regardless of temperature changes. The cartridge transmits the pressure exerted by the bulb directly to the diaphragm. If the evaporator pressure below the diaphragm becomes too high, this will overcome the pressure in the cartridge and the valve will tend to close. When the evaporator pressure falls below the pressure of the gas in the cartridge, the cartridge expands to the original size, transmits normal bulb pressure to the diaphragm and control of the valve returns to normal.

Another method of achieving an MOP threshold is to limit the amount of liquid refrigerant placed in the bulb. In a normal expansion valve there will always be a residue of sufficient liquid refrigerant in the bulb to maintain a mix of saturated liquid and saturated vapour and any increase in temperature at the bulb will bring about an increase in pressure. However, when the amount of liquid is precisely limited, all the liquid will evaporate at higher bulb temperatures leaving only saturated or partially superheated vapour. Under these circumstances, much greater rises in temperature are needed at the bulb to develop higher pressure in order to open the valve. This self-limiting feature is an excellent way of attaining a maximum operating pressure characteristic for the expansion valve. As a guide, it is common practice to select a valve with an MOP of 0.3 to 0.7 bar above the normal evaporating pressure to ensure that the pressure limiting feature is activated soon after excessive evaporator pressure is encountered.

 

Table 1
 

 


Low temperature applications

A study of the pressure-temperature relationship of HCFC22 in Fig 3 shows how the rate of change of pressure diminishes in relation to temperature as the temperature is reduced. Table 1 shown below for HCFC22 illustrates this. Clearly, a greater amount of superheat is required at low temperatures to bring about a change in bulb pressure than is required at high evaporating temperatures. If the evaporating temperature were 5°C, an increase in superheat temperature of 5K will result in a pressure increase of 0.97bar (5.76 - 4.79). However, if the evaporating temperature were 40°C, an increase in superheat temperature of 5K will result in a pressure increase of only 0.27bar (0.31 - 0.04). This is a common characteristic of many refrigerants.

 

Figure 3: Pressure/temperature diagram for R22
 

 


Cross-charged expansion valves

Clearly, in low temperature applications, too much superheat is required to operate the valve resulting in a loss of valuable evaporator surface area and capacity. In order to avoid this situation, a cross-charged expansion valve bulb can be fitted in which another fluid is used instead of the system refrigerant type. This is normally a fluid with a much lower saturation temperature than the refrigerant used in the system to ensure adequate pressure change with temperature at the desired operating conditions. The term cross-charged arises from the fact that the pressure-temperature curve of the system refrigerant is 'crossed' by the pressure-temperature curve of the fluid in the bulb as illustrated in Fig 3.

Capacity range

There are limits to level of modulation to refrigerant flow that an expansion valve can satisfactorily control. In many systems there is a single, fixed-speed compressor and the expansion valve ensures that the correct refrigerant mass flow rate is delivered to the evaporator to meet the instantaneous and constantly varying thermal load. If the thermal load in an application is likely to fall to low levels, good practice demands the consideration of a split evaporator coil with an expansion valve for each circuit. Care must therefore be exercised in selecting the correct valve for the operating range of the application.

NEXT MONTH: Part 12 - Automatic and electronic expansion valves

 

 

With thanks to Mike Creamer of Business Edge who revisits his Masterclass series of articles, updating and adding to the information which proved so useful to readers when the series was first published over ten years ago. In this reincarnation, the series will cover both air conditioning and refrigeration and serve as an on-going source of technical reference for experienced personnel as well as providing a solid educational grounding for newcomers to our industry.

 

1 November 2006

Comments

Shruti
16 October 2014 05:38:34
Dave is correct here. The only way you get more pusesrre inside a water heater than what the water pump or system is exerting, is if the water gets to a temp that makes it near to the boiling point, where it would turn into steam. That's what pusesrre this valve would release. It's actually a combo pusesrre/temp. thing. Although they sometimes start leaking from a lime, calcium buildup in the water supply that has eroded part of the seat on which the valve stem seals. If turning the temp back down doesn't help, it's not rocket science to drain a bit of water off the top and replace the valve. They cost about $8. Was this answer helpful?
Hasmukh Desai
03 September 2014 19:41:43

i check the system with nitrogen testing please tell me pressure go thu electronic expansion valve how is it equal  or how to operate when put pressure in system.

Already Registered?
Sign In
Not Yet Registered?
Register

Thermo King achieves zero waste to landfill

The Galway-based manufacturing facility of Thermo King has become one of Ingersoll Rand's first sites to achieve zero waste to landfill. 

  18-Aug-2017

Danfoss to host Refrigerant Week 2017

Danfoss is organising a Refrigerant Week, putting refrigerants in the spotlight and aiming to educate and inspire professionals.

  15-Aug-2017

Strut Direct

The StrutFoot™ Flat Roof Support System, offering total flexibility for design layout & installation.
  17-Aug-2017
ACR News is the number one magazine in the air conditioning and refrigeration industry. Don’t miss out, subscribe today!
Subcribe to ACR News

Jobs

Area sales person – Southern England

Thermofrost Cryo Plc is a leading national distributor of refrigeration and air conditioning equipment, with offices in London, Birmingham and Manchester.

Diary

Compressors 2017 - 9th International Conference on Compressors and Coolants
10th International Conference on Compressors and their Systems
Cool Talks Breakfast Briefing
The Ins and Outs of Adiabatic Cooling
The IOR Northern Branch Annual Dinner