Hot Gas Bypass

[Moses1]

Hello eveyone,

I'm on a new project in which customer wanted a Hotgas bypass for not stopping compressor when thermostat is ON. I won't use bypass for defrost but just to keep compressor running even if the room is in desired temperature.

What would happen if i add a bypass line to Evaporator inlet and let the thermostat control the solenoid valve? In my plan when the thermostat reaches to desired temperature then solenoid will open. Do i need an additional HGBV? As i told i don't need bypass gas for defrost as my conditions are not so low.

System is with below conditions;
Its an air conditioning system
System will work between 56 F to 122 F
Compressor semi hermetic
Capacity 4.5 kW maximum
Valve block expansion valve 1.5 tons
Gas R134a 1.8 kg.

Thank you very much for your time

[RANGER1]

Moses1,
I would think it may be ideal to have unloader's on compressor if possible.
Also not sure about turning off liquid line solenoid valve, I would leave it on all the time.
You could inject hot gas into inlet of evaporator utilising a hot gas valve which has downstream pressure pilot function. This valve could be called a false load valve in your case.

Set up could be suction pressure decreases, compressor unloads, when suction pressure decreases more, false load valve then opens to give enough load for compressor to continue to operate.

A high discharge temperature safety thermostat could be installed to protect compressor if it goes out of range for the various reasons.

Will be interested on other's comments.

[NewmanRef]

Hi Moses,

You said your customer wants a hot gas line to prevent compressor cut out with thermostat ON. Is the evaporator coil located outside in an AHU or ducting? If so the purpose of the hot gas line is usually to keep low side pressure up at low ambient temp to prevent compressor cutting out on LP before reaching temperature set point.

Are you sure the customer wants the compressor to run with thermostat OFF as well? If so I am interested to know why?

[RANGER1]

This type of control is usually for chilled water, the way I have described it anyway.
Compressor could unload on temperature instead of pressure.

[Moses1]

Hi Moses,

You said your customer wants a hot gas line to prevent compressor cut out with thermostat ON. Is the evaporator coil located outside in an AHU or ducting? If so the purpose of the hot gas line is usually to keep low side pressure up at low ambient temp to prevent compressor cutting out on LP before reaching temperature set point.

Are you sure the customer wants the compressor to run with thermostat OFF as well? If so I am interested to know why?

Hello,

The customer just wants the compressor to run continuous because they think reliability of the compressor would be reduced dramaticly if not. What's your idea about this? If the compressor stops with thermostat and starts again in a few minutes and this repeats for several times, how bad would the compressor affect from this?

I was thinking to do this job without HGBV but just with a solenoid located on HGB line.
My plan was to open the solenoid when the temperature has reached the desired temperature.
The Evaporator is just 3 feet upside of the condenser and compressor.

[FreezerGeezer]

Well ideally the compressor isn't going to have to start frequently (where the most danger is) so I don't see how running it continuously is going to do anything else than wear it out faster?

[Moses1]

Well ideally the compressor isn't going to have to start frequently (where the most danger is) so I don't see how running it continuously is going to do anything else than wear it out faster?

How do you tell it? We don't use time relay to delay compressor start. When temperature is desired the compressor goes off, when the temperature is 2 Celcius more than desired temperature the compressor is on. So the cabins heat will be 2 celcius more in less than 3-5 minutes depending on outside air temperature.

[FreezerGeezer]

There's your challenge, in my opinion. I was always taught that you must have an absolute maximum of 6 starts per hour. You're suggesting about double that, effectively short cycling the compressor.

I understand what you're trying to achieve. Basically, I always thought that Trane's motto 'It's hard to stop a Trane' refers in part to their philosophy of "once the compressor has started, keep it running as long as practical". All their chillers (and probably most other manufacturers as well?) are designed with load / unload systems so that as the set point is neared, the compressors 'back off'. This helps to minimise the number of starts and maximise the starts:run hours.
As Ranger1 said, why not use a compressor with unloaders built in? Or, could you build a very small chilled water system with a buffer vessel? That'd give your compressor a good load to pull down, and the relatively large store of chilled water would allow a reasonable 'off' time for the primary refrigeration system. Perhaps there are other (potential?) cooling loads on site that could be added to the project, the extra beat load of which would keep the compressor running longer.
Or maybe there's some waste 'cool' on site that you could use instead of a vapour compression system?
Or, could you use a swamp unit?