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Common conversion factors

1 ft = 0.3048 m = 30.48 cm

1 in = 2.54 cm = 0.0254 m

1 m = 3.2808 ft = 39.37 in = 1.0936 yd

1 km = 0.62137 mile

1 mm = 1000 microns

1 micron = 1/100000 m = 1/10000 cm

1 kg = 2.205 lb

1 lb = 453.592 gm = 0.45359 kg

1 gm = 0.002205 lb = 15.432 grain

1 slug = 32.1739 lb

1 in sq.= 6.4516 cm sq.

1 ft sq.= 929.03 cm sq. = 0.0929 m sq.

1 cm sq. = 0.165 in sq.

1 m sq. = 10.7693 ft sq. = 1.196 yd sq.

1 in cube = 16.387 cc

1 liter = 1000 cc = 61.024 ft cube

1 gallon = 4.546 liters = 0.1605 ft cube

1 m cube = 35.3148 ft cube

1 ft cube = 0.028 m cube

1 gm/cc = 62.33 lb/ft cube

1 lb/ft cube = 0.016 gm/cc = 16 kg/m cube

1 liter air = 1.2982 gm at NTP

Deg F = (1.8 >< deg C) + 32

Deg C = (Deg F – 32)/1.8

Deg K = deg C + 273.15

1 Bar = 750.06 mm Hg = 401.85 in water

1 Bar = 0.98692 atm = 14.504 lb/in sq.

1 Bar = 100000 kg/m-sec sq = 1000000 kg/cm-sec sq.

1 N/m2  = 1 pascal = 1/100000 bar

1 N/m2  = 1 kg/m-sec2 = 10 g/cm-sec2

1 N/m2  = 1.4504/10000 lb/in sq.

 

Thermal conductivity

1 watt/m-K = 0.5778 btu/hr-ft-deg.F

1 watt/m-K = 0.8598 k cal/hr-m-deg C

1 watt/m-K = 2.309 >< 10-3 cal/sec-cm deg K

1 K cal/hr-deg C = 1.1623 watt/m deg K

1 K cal/hr-deg C = 0.6719 btu/hr-ft-deg F

 

Heat transfer coefficient

1 watt/m sq deg K = 10-4 watt/cm sq-deg K

1 watt/m deg K = 0.86 k cal/m sq-hr-deg C

1 cal/m sq-hr-deg C = 4.184 >< 104 watt/m sq. – deg K

1 btu/ft sq.-hr-deg F = 506782 watt/m sq.- deg K

1 btu/ft sq.-hr-deg F = 4.883 kcal/m sq-hr-degC

1 kcal/m sq-hr-deg C = 1.163 watt/m sq-degK

Methods of checking steam condenser performance

It is desirable to get a rated load or some agreed-upon load on the turbine which will be the same for each successive check and read the load, air leakage, inlet water temperature, outlet water temperature and the absolute pressure in the condenser, and convert the absolute pressure into the corresponding saturated steam temperature; also, calculate the temperature rise, initial temperature difference and terminal difference.

The terminal difference is the difference between the steam temperature and the outlet water temperature; the temperature rise is the increase in the CW temperature. The initial temperature difference is the difference between the inlet water temperature and the steam temperature (or the saturation temperature corresponding to the absolute pressure).

If this data is recorded periodically and checked, any deviation will give the operator the best indication of what has been happening to his condenser.

During a period of high air leakage, when air blankets tube surfaces, the absolute pressure, air leakage, steam temperature and terminal difference will rise and again upon correcting the leakage, will return to normal. Also, during a period of dirty condenser tubes, the absolute pressure, steam temperature and terminal difference increases and after cleaning will return to normal. This holds true with non-condenser type heat exchanger.

Other factors:

Other factors, which would affect the condenser trend, are the change in water inlet temperatures and change in loads. These changes do not affect the condenser performance although they change most of the condenser temperature values and do change the condenser backpressure. As a guide to condenser performance the terminal difference gives the operator the alarm and should be watched carefully.

Data taken at a different lad may be compared to that at the desired load by the following device. Load to the condenser, either in terms of BTU per hour or kw load plotted against rise and initial temperature difference. “Rise” will be a straight line from zero at no load to maximum rise at full load. Disregarding the effects of air leakage and vacuum pump capacity and some of these matters in the very low load range, the initial temperature difference will also be a straight line.

Operators should watch pump discharge pressures and pump horsepower for clues as to tube sheets plugged by rubbish accumulation. These indications can also be obtained from delta P of CW across the condenser.

Poor heat transfer due to tube fouling will affect vacuum performance at all loads but will be most noticeable at high loads.

Circulating water pump data to approximate water flow, which with rise in circulating water temperature gives another approximation of heat load.

The condenser performance is evaluated, expressed as percentage:

 % = U actual >< 100

      U designed

U actual can be calculated as:  U = Q/A∆Tlmtd

U actual = actual heat transfer rate, btu/hr/sq ft/deg F Lmtd

Q = duty, Btu/hr

Significance of pressure

The cold and hot fluid inlet and outlet temperature and ΔP of the fluids across the heat exchangers are very significant in determination of the health of that heat exchanger. In condensers ΔP is shown as vacuum. Any increase in ΔP or decrease in vacuum is an indication of fouling or decrease in flow rate of the fluid.

Increase in ΔP for hot fluid when the inlet hot fluid P remains same, across a heat exchanger could be due to fouling on the heat-exchanging surface exposed to the hot fluid. The same applies to CW side. An increase of ΔP with an increase in inlet P is very much possible.

In condensers the decrease in vacuum could be due to fouling on heat-exchanging surface on both or either side. Most probably it will be due to fouling of heat-exchanger surface on CW side. In condensers the decrease in vacuum could also be due to leakage from the heat-exchanging surface or from the surface exposed to atmosphere.

It is important to know the design pressure on both sides (hot & cold). The high-pressure fluid may ingress in low-pressure fluid. Normally the hot fluid pressure is higher than cold fluid pressure.

Change in ΔP

Indication

Increase, in hot fluid

Increase in flow-rate of hot fluid

Fouling on HE surface on hot-fluid side

Decrease, in hot fluid

Decrease in flow-rate of hot fluid

Leakage on hot fluid path.

Increase, in cold fluid

Increase in flow-rate of cold fluid

Fouling on HE surface on cold-fluid side

Decrease, in cold fluid

Decrease in flow-rate of cold fluid

Leakage on cold fluid path.

It is important to know the design pressure on both sides (hot & cold). The high-pressure fluid may ingress in low-pressure fluid. Normally the hot fluid pressure is higher than cold fluid pressure.

Significance of flow-rates

Flow-rates and any change in flow-rates have different effects on Q and U values. Following are the effects:

Changes

Effects

If hot fluid flow rate is increased

    The ΔP of hot fluid across the heat exchanger will increase.

    The inlet P of hot fluid will increase

    The ΔT of hot fluid will decrease

    The ΔT of cold fluid will increase

    The Q value will increase

If hot fluid flow rate is decreased

    Vice-versa of the above effects

If cold fluid flow rate is increased

 

If cold fluid flow rate is increased

    The ΔP of cold fluid across the heat exchanger will increase.

    The inlet P of cold fluid will increase

    The ΔT of cold fluid will decrease

    The ΔT of hot fluid will increase

    The Q value will increase

    The fouling/scaling propensity of Cold fluid will decrease

    The flow induced corrosion/erosion may increase

If cold fluid flow rate is decreased

    The ΔP of cold fluid across the heat exchanger will decrease.

    The inlet P of cold fluid will decrease

    The ΔT of cold fluid will increase

    The Q value will decrease

    The fouling/scaling propensity of Cold fluid will increase Avoid such situations

Condenser, where the hot fluid temperature varies

In some type of heat exchanger, the temperature of both the fluids is varying and there fore logarithmic mean temperature will have to be calculated. The temperature changes of water and hot fluid may be represented graphically by the following figure. During the process the hot fluid will reject heat in the following manner:

(a) To cool down to saturation temperature, (process DC)
(b) To liquefy (process CB)
(c) To subcool the liquid (process BA)

At the same time the water will receive heat and will get heated from tc1 to tc2.

This type of condenser may be assumed to consist of three sections i.e. (i) desuperheater (ii) liquefier (iii) subcooler.

Here if th1, 2, 3 & 4 is known then LMTD of each section can be calculated and average mean temperature difference is approximately given by:

 ΔT m =                                  total heat rejection (HR) (kJ/min)                        (eq. 1.35)

                       (HRD-C/ ΔT m (D-C)) + (HRC-B/ ΔT m (C-B)) + (HRB-A/ ΔT m (B-A))

And then with this LMTD values for Q and/or U can be calculated.

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