Starting a compressor, and receiver questions.

First of all I would like to apologize myself for only asking questions and adding virtually nothing to this forum. I am simply too inexperienced to be able to give sensible answers to all the questions over here.

Well, a little update for the people who followed my earlier threads: Yesterday I have been able to buy an orifice 0X for the Danfoss TEN2 TEV (with 0 °C MOP point) I will use to replace the capillary tube in the next update of my water/methanol chiller (which works flawless at the moment; only the temp fails to drop below approximately -14 °C). I was also able to buy a filter/dryer with 1/4" ODF solder connections. Obtaining these parts costed me several months :( The Dutch government definitely doesn't want me to build my own refrigeration equipment. But I complained already enough about that. Also, the airtight mainboard box with a silicagel desiccant in it works flawless. I haven't seen any ice or condensation forming during all the time I have been using it. The processor itself is happily running at 2+ GHz.

Getting the system running with TEV should not be such a hard job. All I have to do is to thoroughly clean the inside of the evaporator piping, solder all the various parts together, pull a vacuum, and refill with R134a.

There are only a few hurdles left:

- I am unable to obtain a sufficiently small receiver. The smallest one I am able to obtain is a 5 dm³ one (with 1/2" liquid line connections). Way too large for my system. So, I have made myself a receiver from a 20cm piece of 1" pipe, capped on both sides, and 1/4" copper refrigerant inlets/outlet pipes mounted on one side. The refrigerant outlet runs down to the bottom of the 'receiver', so it draws liquid. This 'receiver' provides me with about 75cc (0.075 litres) of liquid refrigerant storage. Since the inner volume of the evaporator pipe is also small (115cc), I thought this very small size receiver would be sufficient. This assumption is based on the fact that the receiver contains more refrigerant in liquid form than what will fit in gaseous state in the evaporator. Thus, even at the highest operating temperature there will be liquid in the receiver, and the TEV will get a decent supply of liquid refrigerant under all operating conditions. Unfortunately I am at work now, and I have not brought my calculations with me.

Am I correct in these assumptions, and is the receiver sufficient?


- To be able to use my current compressor (Danfoss NL11F) with a TEV, I have to use another starting device. Normally this compressor is used in capillary tube systems, so a simple PTC based starting device can be used (LST: Low Starting Torque). This device only works when pressures at both sides of the compressor are approximately equal when starting the compressor. If there is a pressure difference, this starting device does not provide enough torque to allow the motor to start.
There exists another starting relay / start capacitor combination for this compressor, which produces a higher starting torque, and thus allow this compressor to be used with other expansion devices such as a TEV. The only problem is: I am, as usual, not able to obtain this High Starting Torque start device. Even worse: I cannot obtain information on how exactly this device works. I imagine that during start a start capacitor is placed between the start winding and the mains. This introduces a phase shift, which helps the rotor to rotate. After starting, I imagine that the start winding is switched off, and only the run winding of the motor stays energized.

Is temporary switching on the start winding with a series start capacitor indeed how those HST devices work? If so, I can build myself some electronics which does exactly the same. But: what value should the start capacitor be? And how easy is it to damage the start winding of the compressor? I definitely don't want to blow the compressor.

Hi DaBit,

Your assessment of how a start kit works is correct, although it may not be needed if your system will be running non-stop, and when stopped will have a long off period.

I don't design systems, so I can't help you with sizing questions.

Once running, provide the following temperatures so we can help you fine tune it.

Low side:

Evap water in temp
Evap water out temp
Low side pressure or SST
Suction line temp 150mm from compressor

High side:

Cond air in temp
Cond air out temp
High side pressure or SCT
Liquid line temp 150mm from receiver

SCT can be roughly estimated using a piece of cardboard. Cut a 3 inch cardboard circle, and poke a small hole in the center of it. Hold the cardboard disk tightly against the center of the condenser. Insert your temperature probe through the hole, and between the condenser fins.

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Originally posted by Gary
Your assessment of how a start kit works is correct, although it may not be needed if your system will be running non-stop, and when stopped will have a long off period.

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This implicitely means that pressures *will* equalise in a TEV system during a longer period of time. Is this true? And if so, what would be the nescessary 'off' period? This system has the same run cycle as the PC. That means: on for a few hours, then off until the next day.

Quote:

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Once running, provide the following temperatures so we can help you fine tune it.

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I will provide temps, but I cannot give pressures unless they can be derived from a P-T chart for the refrigerant used.
However, before I have everything up and running another few weeks will pass since I lack necessary equipment like a vacuum pump and oxyacetylene torch.

Quote:

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This implicitely means that pressures *will* equalise in a TEV system during a longer period of time. Is this true? And if so, what would be the nescessary 'off' period? This system has the same run cycle as the PC. That means: on for a few hours, then off until the next day.

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With the compressor off, the pressures on each side of the system will depend upon temperature. The condenser cools down and the evaporator warms up. When the temperatures are equal, the pressures are equal. They needn't be entirely equal, just close enough for low starting torque to handle. And you can always add a start kit later if needed.

But this is only true if there is a connection between the low side and high side where refrigerant can flow to equalise pressure. Now, in this system there are 2 of those connections: the compressor and the TEV. Since the compressor contains valves which block refrigerant flow from high pressure side to low pressure side, this route is invalid for pressure equalisation. So, the only point where pressure equalisation could occur is the TEV. This TEV is a MOP type TEV (to prevent serious compressor overloading during pulldown), so it closes completely when the low side pressure rises above the pressure equalling 0 °C for R134a (MOP point 0 °C). At this point, pressure equalisation stops, and there still exists a pressure difference between the low side and high side.

Is this reasoning correct? If not, where does it fail?

For as long as there is liquid refrigerant in the evaporator, the pressure/temperature relationship will hold true. When the liquid is entirely vaporized, then the temperature will rise, but the pressure will not. Whether the pressures truly equalize depends upon the volume of liquid in the evaporator when the TEV closes off. In any case, it should be close enough.

But of course! It seems that it almost takes a sledgehammer to knock my eyes open, but now I got the point. I hope the evaporator is flooded well enough to ensure that both sides of the system contain liquid after shutoff. And if it just doesn't work, I can always add an undersized start capacitor to help the motor a bit, but without risking the start winding of it.

This brings me to another question: normally I use the evaporator in such a way that liquid refrigerant flow is also gravity supported. Thus: liquid refrigerant from the throttling device (currently a captube; will be a TEV) is supplied at the top of the evaporator, and evaporates while it travels to the bottom of the evaporator.

Is this still the best configuration to use in combination with a TEV? Or would it be wiser to supply the liquid refrigerant at the bottom of the evaporator?

Top feed will help return oil, which can be a problem with very low temperature systems.

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Thus: liquid refrigerant from the throttling device (currently a captube; will be a TEV) is supplied at the top of the evaporator, and evaporates while it travels to the bottom of the evaporator.

Is this still the best configuration to use in combination with a TEV? Or would it be wiser to supply the liquid refrigerant at the bottom of the evaporator?

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A bottom fed evaporator coil will often provide more capacity than a top fed coil. This is particularly the case if refrigerant velocities within the coil are low, resulting in some trapping of liquid refrigerant in the coil. But from a TEV performance perspective, a top fed coil is usually the better route if it alleviates trapped refrigerant

When refrigerant gets trapped in the evaporator, it has a tendency to flow thru the remainder of the coil in slugs. This can cause the TEV to hunt.

Regarding if your system will equalize with a TEV, Gary is correct in that pressures will depend upon temperature and the amount of refrigerant remaining in the low side of the system after compressor shutdown. As one might expect, TEVs with MOPs will tend to hold higher pressure differentials than TEVs less an MOP. Also, TEVs are available with internal bleeds for the purpose of equalizing pressures, and have been used for years on residential a/c systems using low starting torque compressors. The use of scroll compressors in this market, however, has reduced the need for bleed type TEVs.

OK, I'll use a top-fed evaporator.

Prof, can you shine a light on the size of the receiver? I suppose even the small size I plan to use is sufficient, but I would like a comment on it from a professional.

How do you determine receiver volume in a system anyway? There must be a guideline for it.