VCRU expansion valve qu

[jcb4vcru]

We are building a Vapor Compression Refrigeration Unit to cool a simulated, scaled-up electronic chip (cold plate with heater assembly on surface). We are trying to determine the best expansion valve to use from the condenser to the evaporator. We are hoping for any easily operated and easily integrated component. Some members believe that a manual valve with 30-40 turns would be the way to go. We are not sure if we have the means to integrate and maintain a thermostatic expansion valve. What would you recommend? The evaporator load is under 600W. Mass flow is 0.0036 kg/s.

Thank you.

 

[lGerryl]

Much more detail would be required to give a definative answer.

There are several options to use as a metering device in your VCRU.
1) hand expansion valve (what you are suggesting)
2) Capillary tube
3) automatic expansion valve
4) Thermostatic expansion valve
5) Electric or electronic expansion valves.

All of the above are used in OEM refrigeration equipment depending on a large variety of system parameters including but not restricted to the following.
initial first cost
stability of load
refrigerant type
evaporator temperature range
skill of the design personal
skill of the construction personal
longevity of the equipment.
etc etc etc....

My personal opinion would be ....
For typical system, with a normal operating ambient and a somewhat variable load, using a non-exotic commercial refrigerant,(excluding ammonia), in a standard temperature range (-20 to +50°F SST), a thermostatic expansion valve would be my initial choice to give the best performance, with the least amount of initial cost and engineering design time.

Your capacity (0.5kW / 3.51 kW/ton = 0.14 tons)
This is a small valve, but they are available.
The next choice would be an automatic expansion valve which will control a constant evaporator pressure (which gives a constant evap temperature), but does not handle variable loads as well.
The operating charateristics of these valves are different and should be investigated before deciding which choice would be better for you.
Hand expansion valve for me would be the LAST CHOICE... and only used if knowlegable personal were available to fine tune the system as it runs. Although for a Research and development type of application used in conjunction with flow meters etc. this may be exactly what you need.

Gerry

 

[jcb4vcru]

We are working with R-134a. The load will not initially be varied, but if time permits and initial experimentation is successful we my attempt to increase the load up, but not higher than .32 tons. We will be implementing a flow meter and are in the process of locating one for our system size. Going into the Exp. Valve we are working with 1/4" flare copper, and 3/8" flare tube at the exit. The design team is all relatively inexperienced. They will also be performing the majority of the construction. Evacuating and charging will be outsourced to a local professional. The load is a simulation. We have a 6"x4" Lytron CP-10 cold plate, on which an epoxy and resistive heaters will be placed. As far as cost goes, we have $500+ left for this part and the flow meter and are applying for some additional grants - and are not overly concerned on cost. Evaporator temp range is currently calculated at 5-50 deg C.

If any other info needed just ask. Based on your final recommendation, what vendors do you know of have valves for this type of load, application, etc.?

 

[dental]

keep it simple use a cap tube

 

[lGerryl]

WOW the eveporator is only 4x6 inches? I cant envision a 1/4 ton of circuiting in that area without a high deltaT.
If the components are not all balanced for the load at a specific evap temperature, perhaps the hand expansion valve would be the most forgiving solution. Some flowmeters in that flow range are equipped with needle valves that may double as the expansion valve.

If the refrigeration unit is a mock up for a production cooling unit, than as recommended by Dental, the best and simplest long term solution may be a cap tube.
If the unit is to be used to test various versions of the scaled up chip, and the loading and temperature range are not exatly known, then a little more versitility in the capacity of the metering device is required.

The cap tube is certainly the simplist device, (used in domestic fridges, freezers, window air-conditions etc.) However, once selected and installed, the capacity of the system basically fixed. Cap tubes tend to overfeed on light loads, and underfeed on high loads so are suited for constant load systems only. Cap tube systems are criticaly charged and the capacity cannot be adjusted without recovering the refrigerant and changing the tube.
The TX valve can modulate between roughly 30 to 100% of its rated capacity.
Hand expansion valves can go from shut off to wide open.

Cap-tubes and Expansion valves are available at any local refrigeration wholesaler. Your local service guy could recommend the store and brand most common in your area.

 

[boyceg]

We use a WW Grainger constant pressure valve on small systems. The constant pressure is maintained at the evaporator. An accumulator is suggested to protect the compressor for low load operation. If the temperature difference between the condenser and evaporator is large, consider a liquid line to suction line heat exchanger.
Boyceg

 

[imok2]

In refrigeration and air conditioning applications, there are two main types of expansion valves: thermostatic expansion valves (TXVs), and automatic, or constant-pressure, expansion valves (AXVs).
The thermostatic expansion valve is a metering device that regulates the flow of high-pressure liquid refrigerant to the low-pressure evaporator in response to the heat load on the evaporator. The valve’s primary function is to maintain maximum evaporator efficiency and protect the compressor by preventing liquid refrigerant from passing through the suction line to the compressor. It does this by controlling the flow of liquid refrigerant into the evaporator, so that it equals the rate at which the liquid is completely vaporized by the heat load on the evaporator and the resulting suction gas is superheated. It controls flow to maintain a predetermined level of superheat.
. The constant pressure or automatic expansion valve responds to outlet pressure. It meters liquid refrigerant to the evaporator, maintaining a constant evaporator pressure and refrigerant flows at a rate that exactly matches compressor capacity. The valve incorporates a diaphragm that separates atmospheric pressure and system pressure.It also has a range spring with an adjustable pressure setting which is located above the diaphragm. Beneath the diaphragm is a stainless steel push rod and ball assembly backed by a closing spring. An o-ring located on the push rod creates a balanced port. This feature balances out the effect of increasing or decreasing inlet pressure, and aids in maintaining a constant outlet pressure in addition an equalizer passageway is incorporated into the valve body. I would use the latter for this application