Heating / Cooling Control

[walden]

There is a demand for close temperature control systems with heating and cooling regulation. Heating is easy! A thyristor current controller and a self-tuning, PID temperature controller will give ± 1°C of the setpoint. Cooling is a problem! With a conventional liquid line solenoid valve, the time taken for the compressor to start and get any cooling effect in the evaporator means that the room temperature is racing upwards. What is needed is to keep the compressor running continuously so that cooling is available immediately it is requested. Refrigeration engineers mumble the words “hot gas by-pass”. If you have any other ideas …

[frank]

For close control applications I find it better to use chilled water

[walden]

Frank, I agree, CW is a very good medium to use for close temperature control. However, it is only available with big companies. The usual scenario is a relatively small company with a single close temperature chamber, no CW on site. In these circumstances, a DX evaporator appears to be the only answer. Hence the question. Being only an electrical engineer, I am hoping for some ideas from you refrigeration wizards out there!

[Brent Hall]

I've used pulsing Hot-Gas for close cooling control to +/- 0.2K on chilled water systems, but only when there is no heating. The problem with heat+cool PID control is that they fight. Its better to over cool and control the temp accurately by heating and have enough heating to substitute the load to keep the (on/off) compressor running. This way, using a Thyristor, you can get very accurate control. It is also better to use chilled water which is easier to manipulate than a DX gas. In any event, once the compreeor stops, you can say good by to close control.

Good luck & up the cherries.

[US Iceman]

In the projects I have worked on where similar requirements are needed for continuous operation hot gas bypass is the only way to go for small to medium applications.

Using hot gas bypass to false load the evaporator and compressor provides continuous operation for close humidity and cooling control. The hot gas is controlled by an outlet pressure regulator which senses the evaporator pressure.

When the evaporator pressure decreases due to a reduction in heat load to the coil, the bypass regulator modulates open to control the evaporator pressure.

The hot gas is usually introduced downstream of the TXV, but before the refrigerant distributor to provide good mixing of the hot gas and liquid (from the TXV).

Hot gas can also be introduced into the suction line upstream of the compressor, but in this case you will also need some liquid refrigerant injection to desuperheat the suction gas before it enters the compressor.

Without the liquid injection, the compressor can overheat due to the high inlet gas temperature.

I also agree with Franks' statement that chilled water works quite well. The mass of cold water being circulated provides a dampening effect for temperature rise.

Depending on the type of load, you might also want to consider a water storage tank to provide sufficient reserve cooling capacity to allow the refrigeration system (chiller) to control the temperature and cooling load.

[rbartlett]

what about inverter ?

cheers

richard

[US Iceman]

what about inverter ?

This another method, but you have to ensure the evaporator liquid feed control is capable of operating at reduced capacity.

At some point if the load reduces sufficiently the inverter will try to slow the compressor down too slow and you can loose oil pressure. So you have some minimum capacity still available.

If the system cooling load is lower than the minimum capacity reduction of the system you can still have problems with short-cycling or loss of temperature control.

[frank]

The usual scenario is a relatively small company with a single close temperature chamber, no CW on site.

Have a look here. There are also other sites that have chillers down to 5kw. http://www.daikin.co.uk/aircondition...s/euwa-kzw.jsp

[phil68]

When I was working on rooms for the storage of pharmaceuticals it called for a stability of +/- 0.1 deg. C. They had refrigeration running 24/7 & heating rated at 50% more powerful than the refrigeration. The heat would be pulsed to maintain the temp. with a self-tune controller (very wasteful from an energy consumption point of view) & the humidity was controlled +/- 5% RH with a small water boiler controlled by a 2nd channel on the controller.

[pradyutghose]

From where I can get the RLA of Kirloskar Copeland sealed compressor?

[pradyutghose]

Sir,
I am not getting proper cooling, if anybody can suggest.
Compressor:1.5 TR
Evaporator : 375 x 375 mm x 4 row
Suction Pressure : 70 psi
Discharge : 290 psi
Ambient : 34 DegC
Air Flow : 350 cfm
Compressor: Copeland make/Hermatic-Sealed
I am not getting coil temperature less than 10 Deg C Why?

[Andy]

Sir,
I am not getting proper cooling, if anybody can suggest.
Compressor:1.5 TR
Evaporator : 375 x 375 mm x 4 row
Suction Pressure : 70 psi
Discharge : 290 psi
Ambient : 34 DegC
Air Flow : 350 cfm
Compressor: Copeland make/Hermatic-Sealed
I am not getting coil temperature less than 10 Deg C Why?

Without air on to the coil conditions its impossible to say, however it seems the cooling is uncapacity, or the compressor is struggling. If the gas is R22 and the head is 290 psig with a ambient of 34deg c it would indicate an under sized cooling system.

Kind Regards, Andy