© Danfoss A/S (RA Marketing/SB/mwa), May 2009 1

| TECHNOLOGY | Regulators for water-cooled condensers

Regulators for water-cooled condensers

Fitters Notes (Part 13)

Stephan Bachmann, Danfoss Refrigeration & Air Conditioning, Offenbach

Four-way reversing valves for heat pumps or air conditioners were the subject of the last issue of Fitters Notes. Now we want to turn our attention to mechanical cooling water regulators. These are valves that are used specifically for water-cooled condensers. Cooling water regulators can be pressure-controlled or temperature-controlled. Their purpose is to maintain a constant condensing pressure in the refrigeration system by adjusting the flow rate of the cooling water.

Water valves – pressure-controlled and temperature-controlled

Construction of pressure-controlled water valves Cooling water regulators have two connections for the water circuit and one connection for the refrigeration circuit. The smaller devices with pipe connections up to 1 1/2”, have female BSP threaded ports, in line with the norm for water systems. For connection sizes of 2, 3 and 4 inches, flange connections are standard; they have to be butt-welded to the pipe. The connection for the high-pressure line of the refrigeration system is a 7/16 UNF flared nipple fitting. It can be connected to the refrigeration system by 6-mm copper tubing or a prefabricated capillary tube with flare nuts.The WVFX model in size 25 has a grey condensing pressure adjustment knob on the side opposite the refrigerant connection. Turning this knob in the ‘+’ direction (anti-clockwise) increases the setting and thus raises the setpoint condensing pressure (or the condensing temperature, since the pressure and temperature have a

fixed relationship in the wet-vapour region of the condenser). For example, if you want to raise the condensing temperature with R404A refrigerant from 35 to 40 °C, you should turn the adjustment knob in the ‘+’ direction (anti-clockwise). Turning the knob in the ‘–’ direction has the opposite effect: it lowers the condensing pressure or condensing temperature.

WVFX and AVTA setting

Construction of temperature-controlled water valvesThe operation and setting of these valves are exactly the same as with their pressure-controlled counterparts. The main difference is that this regulator responds to the actual value of a temperature.For this purpose, temperature-controlled cooling water regulators (such as AVTA) have a remote sensor that measures the actual temperature. This means that with an ATVA valve as well, the adjustment must be turned in the ‘+’ direction (anti-clockwise) to raise the setting (such as from 35 to 40 °C).

AVTA temperature-controlled cooling water regulator

Sensor locationIt isn’t especially easy to measure the condensing pressure with a temperature-controlled device. With the pressure-controlled version (WVFX), for the refrigerant connection any desired location can be chosen for connecting the far end of the control line to the high pressure side (hot gas line or liquid line), but this is not possible with the temperature-controlled version (AVTA). For example, the temperature that is sensed if the remote sensor is put onto the hot gas line is not the condensing temperature, but instead a significantly higher temperature.This is because the refrigerant at this point is fully gaseous and superheated. Consequently, it is not a suitable location for attaching the AVTA sensor. The situation is similar with the liquid line, because there the refrigerant is subcooled. The temperature there is thus lower than the temperature read from the high-pressure gauge as the temperature corresponding to the indicated pressure. The condenser outlet is a more suitable

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| TECHNOLOGY | Regulators for water-cooled condensers

location, but the sensor should be placed ahead of the receiver.Aside from the difficulties described above, it is also necessary to ensure good heat transfer between the refrigerant line and the sensor. This requires special attention if the sensor is fitted to a stainless-steel tube. Stainless steel is a poor conductor of heat. As a result, the temperature indication received from the sensor by the AVTA cooling water regulator may lag slightly behind the actual temperature. This increases the inertia of the control system, which in many cases causes fluctuations in the condensing pressure.It is normally not advisable to fit the sensor to the cooling water line itself, since it is generally preferable to have the sensor measure the temperature of the medium to be cooled.If an AVTA regulator is actually used to maintain the cooling water at a constant temperature, it is essential to pay attention to the following: to enable re-opening of the valve after it closes, a bypass should be fitted across the cooling water regulator.Otherwise there is a risk that the temperature-controlled cooling water regulator will not open again because the temperature at the sensor location is low (no need for opening), and the flow of warm water is also blocked.

Temperature range The temperature range is also an important factor in the selection of a temperature-controlled cooling water regulator. AVTA and WVTS devices are available with temperature ranges of 0 to 30 °C, 25 to 65 °C, and 50 to 90 °C. The AVTA is also available with a special range of 10 to 80 °C. For standard refrigeration systems with condensing temperatures between 30 and 55 °C, the AVTA or WVTS version with a range of 25 to 65 °C is recommended. In theory, the 10 to 80 °C range could be used, since this would also cover the normal condensing range. However, in practice the 25/65 version is preferable here because the regulator has better regulation resolution due to its smaller temperature range. If a temperature regulator with a sensor in a sensor pocket has to be replaced, you also have to pay attention to the diameter of the sensor. Versions with diameters of 9.5 and 18 mm are available.

From the above discussion, you can see that pressure-controlled cooling water regulators are fundamentally more suitable than temperature-controlled versions for use in compression refri-geration systems. The latter type is more suitable for special applications, such as systems using special refrigerants with relatively high operating pressure.

Location of the valve in the system

Location of the cooling water regulator Questions often arise about the location of the cooling water regulator. It can be fitted in the water circuit either ahead of the condenser or after the condenser. In addition, a coarse particle filter should always be fitted in the water system ahead of the cooling water regulator.Its purpose is to filter out relatively large particles of foreign matter present in the water system. Coarse particle filters must be serviced regularly. The service interval depends on the degree of contamination of the cooling water.As a rough guide, you can assume a six-month service interval. If conditioned river water is used, as often happens with large companies near a river, an even shorter service interval may be necessary. Special versions of the WVFX 10 - 25 in stainless steel are also available for use with aggressive media that would corrode the housing of standard cooling water regulators.Addition of antifreeze or use of a brine solution may be necessary when the system is not operating during the winter, in order to prevent freezing of the cooling water.

Design chart

Pressure drops The water volume flow rate and size of the cooling water regulator are significant factors in the dimensioning of temperature-controlled and pressure-controlled cooling water regulators. They result in a certain pressure drop, which must always be taken into account in the selection process of valve sizes..In case of direct-operated cooling water regulators such as the pressure-controlled WVFX model or the temperature-controlled AVTA model, primary attention should be focussed on avoiding excessive pressure drops, since both types of valves can operate stably even with extremely small pressure drops.With servo-operated cooling water regulators such as the WVS (pressure) and WVTS (temperature), attention must be given to the minimum pressure drop as well as the maximum pressure drop. For example, the WV(T)S requires a minimum water pressure drop of 0.3 bar for stable operation.If a value less than this is given in the selection process, a valve with a smaller rated capacity must be selected. Otherwise the valve will have unstable control behaviour in operation.Excessive pressure drops should be avoided in pump-driven systems. As a guideline, the pressure drop should be well below the 1-bar level. It is recommended to use a adjustment valve to prevent operation of the pump against a closed two-way valve (the cooling water regulator). When the valve is closed, the adjustment valve allows the water to flow back to the pump via a bypass.This ensures a continuous flow of water and avoids damage to the pump. If the water ahead of the valve comes from the city water system (with a water pressure

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| TECHNOLOGY | Regulators for water-cooled condensers

of 4 bar, for example) and it is discharged freely after the valve, a significantly smaller valve can be chosen.

In practice, the pressure drop across the valve is always 4 bar in this case.

WVFX pressure-controlled water valve, high-pressure version

WVFX cross-section (pressure-controlled)

Refrigerant compatibilityGenerally speaking, pressure-controlled cooling water regulators are suitable for use with all common HFC and HCFC refrigerants if the control range and the maximum working pressure matches the design criteria of the overall system. For example, a WVFX 15 valve has an maximum working pressure of 26.4 bar for the refrigerant connection and is thus suitable for use in a system with R407C refrigerant and a maximum working pressure of 25 bar. With a setting of 40 °C (dew-point temperature) on the high-pressure side of the refrigeration system, this corresponds to a gauge pressure of approximately 14 bar. The WVFX 15 is available with two different control ranges: 3.5 to 16 bar gauge pressure and 4 to 23 bar gauge pressure.The version rated at 3.5 to 16 bar gauge pressure is suitable for the case described here. With the same system operating at 50°C condensing, the version rated at 4

to 23 bar gauge pressure would be the right choice, since a condensing dew-point temperature of 50 °C corresponds to a working pressure of approx. 19 bar. However, the latter example is only theoretical, since systems with water-cooled condensers are usually operated with lower condensing temperatures than air-cooled systems. Condensing temperatures in the range of 30 to 40 °C, including in the summer, can be regarded as standard for water-cooled systems. The value with air-cooled systems is often 10 K higher.There are also special versions of the WVFX 10 - 25 for use with especially high pressures. They have a maximum working pressure of up to 45.2 bar and working ranges of 15 to 29 bar positive pressure. These devices are suitable for use with R410A or R744 (CO

2) refrigerant

in subcritical operation.

Refrigerant and water pressureWhen selecting a pressure-controlled cooling water regulator, it is important to note that pressure specifications in datasheets can refer to the water system or the refrigeration system. When you are checking whether a WVFX or WVS regulator is suitable for a particular condensing pressure or maximum working pressure of a refrigeration system, you must use the values in the ‘Condensing side’ section. This is where you find the refrigerant pressure specifications.However, if you are interested in the pressure drops in the water system or the maximum working pressure in the water system, the relevant information is located in the ‘Liquid side’ section.

WVFX 10 15.0 → 29.0 45.2 60.0 16 24 1.4WVFX 15 3.5 → 16.0 26.4 29.0 16 24 1.9WVFX 15 4.0 → 23.0 26.4 29.0 16 24 1.9WVFX 15 15.0 → 29.0 45.2 60.0 16 24 1.9WVFX 20 3.5 → 16.0 26.4 29.0 16 24 3.4WVFX 20 4.0 → 23.0 26.4 29.0 16 24 3.4WVFX 20 15.0 → 29.0 45.2 60.0 16 24 3.4WVFX 25 3.5 → 16.0 26.4 29.0 16 24 5.5WVFX 25 4.0 → 23.0 26.4 29.0 16 24 5.5WVFX 25 15.0 → 29.0 45.2 60.0 16 24 5.5WVFX 32 4.0 → 17.0 24.1 26.5 10 10 11.0WVFX 40 4.0 → 17.0 24.1 26.5 10 10 11.0WVS 32

CFC, HCFC, HFC

R717 (NH3)

2.2 → 19.0 26.4 29.0 10 16 12.5WVS 40 2.2 → 19.0 26.4 29.0 10 16 21.0WVS 50 2.2 → 19.0 26.4 29.0 10 16 32.0WVS 65 2.2 → 19.0 26.4 29.0 10 16 45.0WVS 80 2.2 → 19.0 26.4 29.0 10 16 80.0WVS 100 2.2 → 19.0 26.4 29.0 10 16 125.0

CFC, HCFC, HFC

Type

Condenser side Liquid side kv value

m3/h

Refrigerant

Control press.

adjustable closing press.

bar

Max. working pressure

PBbar

Max. test

pressurep’

bar

Media

Max. working pressure

PBbar

Max.test

pressurep’

bar

Fresh water, neutral brine,

sea water

Fresh water,neutral brine

Preview This completes our discussion of cooling water regulators, but not our discussion of the options for regulating condensing pressure. In the next issue, we will examine speed control of condenser fans using phase-cutting devices.

XGE and RGE condenser fan speed controllers

Refrigerant- and water pressures